Well-Supplied Underground Gas Storage Controls G. SZECSO˝ (2005)

INTRODUCTION

about 3000 ft below the ground, it is about 300 ft thick, and it is penetrated by hundreds of natural gas wells, grouped in two distinct regions, referred to as the Northern and the Southern sites. The storage facility is operated in two cycles. During the winter, it is in its production cycle (PRC), during which the natural gas is removed from storage and is distributed to the users at between 725 and 870 psig (50 and 60 bars) pressure. The pressure difference of 1450 to 870 psig is used up as pressure

This section describes the controls of an actual underground natural gas storage system and the associated distribution network that supplies the nation of Hungary with natural gas. This large operating facility takes the gas from several natural geographic layers of sandstone, pressurizes it to 1450 psig (100 bars), and stores it in a spongy, underground sandstone formation (Figure 8.41a). This underground storage layer is

Wellk 1087 PSIG PT PT 3 3

Cave system

...

Spongy sand stone Northern site

FIC 2

FIC 2

P INC PRC

...

CV−2

INC - Injection cycle

Wellj

CV−1 Ground level

PI

Spongy sandstone Southern site

725−870 PSIG PT 4

...

SIC

Zero-point

PC

* At the end of PRC only

Make-up

PCC – Pressure correction ... – Multiple units

CV−2

* Booster * compressors Const.

M

FIC 1

From LDA

From LDA

users

PCC From Pressure tight LDA bedrock

MCH - Main collection header

PI

From PC FIC CV−1 C 1 IDA Separator ... Separator

FIC 1

PRC - Production cycle

...

Welli

CV−1

N 300'

Cave system

PI ...

N 3000'

Ground level

From LDA

MCH

Flow 1450 LDA FIC PSIG Well1 PI

Gastight bedrock

(2005)

N 300'

B. G. LIPTÁK

N 3000'

8.41

q1 = FTQ

Gas compressors

M

Injection mode Production mode LDA Load distribution algorithm generated set point

From FIC LDA

SIC

FIG. 8.41a Schematic representation of an underground natural gas storage facility. It supplies a nationwide natural gas distribution system in the winter, when it is operated in its production mode (PRC). The actual facility consists of 100+ wells, a number of dryers, compressors, and the associated network of piping. Each well connects to a storage area of spongy sandstone. In order not to damage the underground geological formations, the differential pressure between storage areas served by the neighboring one should be minimized.

2194 © 2006 by Béla Lipták

8.41 Well-Supplied Underground Gas Storage Controls

drop through the well flow control valves (CV-1s in Figure 8.41a), the dryer flow control valves (CV-2s), the dryers, and the connecting system piping. At the end of the production cycle, a booster compressor station can serve to remove the remaining low-pressure gas from storage. After the production cycle of the winter, the depleted wells are refilled during the injection cycle (INC) of the summer. During this cycle, compressor stations deliver fresh natural gas, obtained from domestic gas containing sandstone layers in the area (and from gas supplied mainly by Russia), into this underground gas storage (UGS) system. Figure 8.41a schematically describes the overall UGS system. The reader is reminded that the controls for the various unit operations that are utilized in this system are discussed in more detail in the following sections: Section 8.15, Compressor Control and Optimization, Section 8.22, Dryer Controls, and Section 8.28, Header Pressure Control. It should also be kept in mind that what is being described is an operating system that has evolved from a manually controlled process that has been in opreation for nearly a decade.

THE PROCESS Over millions of years, hermetically sealed multiple hydrocarbon gas layers evolved in the sandstone under Hungary. This region used to be the sea bottom of the ancient Pannon Sea. The natural gas in these layers is extracted and stored in a pressurized artificial storage layer to serve as the gas energy supply of the nation. The shape of the storage layer is elliptical, about 300 ft thick, and is situated about 3000 ft below the surface. The gas is collected by hundreds of wells, which are grouped at two gas-collecting stations (Northern and Southern sites). The wells are periodically discharged (production mode of operation) and then refilled (injection mode of operation). Piping is provided among the wells and stations, serving the purposes of flow and pressure control. The two stations are also connected by one large-diameter pipe. At one of these stations, the natural gas is dried and conditioned during the PRC so as to be suited for longdistance transportation. During the reinjection cycle, the natural gas is distributed for reinjection in order to refill the wells during the INC. The whole system is comprised of more than 100 gas wells. The UGS system therefore consists of the Southern and Northern sites and the above-ground plants, which include the distribution pipelines, separators, dryers, and compressors. The gas supply is divided 60 to 40% between the Southern and the Northern sites, where the majority of above-ground equipment is also located. All gas wells and the above-ground facilities been designed for 1450 psig (100 bars) operation. The gas distribution network is designed for 870 psig (60 bars). If the adequate conditions

© 2006 by Béla Lipták

2195

exist then the above-mentioned two systems will be connected to each other. The underground gas storage system has two modes of operations, which mostly depend on the seasons. The control system in both modes of operation has to provide protection for the natural geological formations and the sandstone storage, while providing continuous operation either in the injection or the production mode of the underground storage system. The Operating Modes The two modes of operation are referred to as the pressure correction control method (PCCM) and the load distribution algorithm (LDA). In both cycles, unique algorithms perform the controls of the process. Digital controls (DCS) are used to implement the algorithms that generate the control signals to the manipulated variables. Basically, the system has a start-up mode, TWM1 (Technological Working Method #1), during which the controls start the process and supervise the changing operating set points (OP), and TWM2, which stabilizes the operating points (OP) and maintains it in the steady state. During the start-up phase (TWM1), the gas supply to the nationwide distribution network is not stable or tightly controlled. Table 8.41b gives a summary of the two modes of operation. Injection Cycle In this mode of operation, single automatic stage flow control (SASFC) is provided with the control loop in automatic (ACM) for the individual gas wells in order to optimize the operation of the whole group of wells. In this mode of operation, the load distribution algorithm (LDA) provides the flow set points for the flow controllers on the individual wells.

TABLE 8.41b The Two Modes of Operation of the Underground Gas Storage System Operating Modes (Cycles)

During the Injection Cycle (INC)

During the Production Cycle (PRC)

Start-up mode: Technological Working Method #1 (TWM1)

Starting process and groups of compressors. Supervising the operating set points (OP).

Starting process for operation in the production cycle (PRC). Supervising the operating set points (OP).

Normal operating mode: Technological Working Method #2 (TWM2)

Stabilizing the operating set points (OP). Supervising gas distribution among wells.

Stabilizing the operating set points (OP). Producing required gas quantity at the required outlet pressure.

2196

Control and Optimization of Unit Operations

During the start-up mode of the INC, the flow into the individual wells is limited by the maximum flows that are allowed for each well. These limits are used in initiating the emergency stoppage of the compressors or the changes of OPs. One of the goals of the control system is to have the pressure in the main collection header (MCH) approximately approach the average pressure of the underground gas storage (UGS) system in order to minimize the energy consumption of the compressors. This is achieved by the use of weighing factors (WFs), so that the load distribution will be proportional to the capacities of the individual wells. The compressors are manually controlled, and it is important that the manual changes that modify the injected total gas flow set point do not upset the stable operation of the cascade flow control on the individual wells. When the compressors are restarted after a power failure, they are protected against overpressure. All other control and safety functions, including control mode switching and emergency responses, are the same as used during the PRC, which is described below. Production Cycle In this mode of operation, single automatic stage flow control (SASFC) set points are provided for the single flow loops on the individual wells and also for the wells that are provided with LDA cascaded flow control. The LDA serves to guarantee that the total gas produced matches the total demand. The flow set points of the individual gas wells are set according to their WFs with minimal actuator actions. During the operation of the system, it is necessary to limit the pressure drop across the control valve (CV-1 in Figure 8.41h) between the wells and the MCH and also on the dryers (CV-2 on Figure 8.41k) to 290 psid. When the production OP is steady, it is the goal to limit the error between the actual and the desired flow (Q) to ± 1%. It is desirable to keep the control system operable even if the measurement errors of the ultrasonic flowmeters on the gas w wells (F in Figures 8.41h, j, and k) increased or if deep groundwater interfered with their operation. It is also desirable that the upset caused by the manual starting of the booster compressor station should not interfere with the operation of the other control loops. MODELING AND SIMULATION The simulation of the operation of the total system requires the modeling of the individual equipment components, which are listed in Table 8.41c. The overall system model is composed of several submodels. The first submodel is the model of the individual wells (GGW1, GGW2, and so on), which includes the models of the valves, sensors, and their PI controllers. The typical operating conditions of a well are listed in Table 8.41d.

© 2006 by Béla Lipták

TABLE 8.41c Submodels Are Required of Each System Component in Order to Model the Total System System Component

Function

Control valve (CV-1)

Control valve with positioner* and its programmable characteristic curve, which controls the gas flow from each individual well

Butterfly valve (CV-2)

Butterfly valve with positioner* to control the flow through the dryers

Flowmeter w d (F and F )

A sonic, gas flow-measuring block with programmable timing from 5 to 60 readings

Main collection header (MCH)

The model for the main collection header is a block that is a nonconcentrated and nonlinear, long-distance pipe model

Dryer

The model representing a dryer block with variable capacity

Well group

The model of three groups of gas wells

Dryer group

The model for a group of dryers

Controller (PID)

Proportional and integral control** algorithms for the flow controls of the gas wells and the dryers

Load

A variable load change model for the zero point

* Generally we do not use positioners on flow control valves, because flow is a fast process and the positioner cannot keep up with it, which results in detuning of the controller and in limit-cycling and hunting. ** In cascade configurations, external reset is required to protect against integral windup, when the loop is switched from cascade to manual or direct slave control.

The other three submodels are that of the MCH, the group of dryers (GD), and the simulated load change (LC). While the model of the gas wells (GGW1, GGW2, and so on) can use real, dynamically measured data, the other three

TABLE 8.41d Typical Operating Conditions Used in Modeling the Wells Parameters

Conditions

Gas temperature

T = 50°F

Set point (reference signal) change

∆q = 1310 yd /h

Set point (work point) at steady state

qst = 3270 yd /h

3

3

Average system pressure

943 psig

Sampling time of flow detection

15 sec*

PI controller tuning parameters**

PB = 150%, Integral time = 120s/repeat

* The sampling time on flow loops should be much shorter than 15 sec. Also, because of their noisy nature only positional algorithms should be used. ** These tuning parameters can be improved (PB narrowed, integral time shortened), if the valve positioner is eliminated.

8.41 Well-Supplied Underground Gas Storage Controls

2197

Sensor Selection GGW1

MCH

GGW2

It is important that when two flowmeters operate in series, such w as the flow sensors at the wells (F in Figure 8.41h) and at the d dryers (F in Figure 8.41i), they be calibrated together, so that they will give the same readings at the same flow rates. It is also important that the sampling time of the flow sensors be short enough so that their contribution to the flow control loop dead time is minimum, because otherwise the flow control loop will cycle. It is also important that only smart, self-diagnosing sensors be used, and only those suppliers be considered that have substantial experience with natural gas service applications. As can be seen from Table 8.41f, the number of ultrasonic flowmeter suppliers is large, but as of this writing the most experienced are Daniel and Instromet.

GGW3

GD

LC

FIG. 8.41e Functional block diagram of an integrated gas storage model, where GGW1–GGW3 are the submodels of the gas wells, MCH is the submodel of the main collecting header, and GD is the submodel of a group of dryers and represents a simulated load change (LC).

submodels utilize only statically measured inputs. Figure 8.41e illustrates the relationship of these submodels within the total model of the gas storage system model. In order to decrease dynamic and static deviation, nonlinear control needs to be used. Capacity of the gas well has a long-term (monthly) and a short-term (hourly) characteristic. In either case, the pressure drop across the control valve (CV-1 in Figure 8.41h) affects the dynamics of the flow control loop. For this reason, adaptive control is desired to adapt the tuning of the loop to the changing inlet pressure to the valve. The effect of the variations in outlet pressure can be minimized by pressure compensation. GENERAL CONTROL CONSIDERATIONS The quality of control can only be as good as the sensors, the control valves, and the control algorithms used. Therefore, it is desirable to carefully consider their requirements.

© 2006 by Béla Lipták

Control Valve Selection The proper selection of control valves is equally important. On fast loops such as flow control loops, the use of positioners is rather questionable, because in effect the positioner is the cascade slave of the flow controller, and therefore its time constant should be one tenth of its master, and such highspeed positioners are seldom available. A controlled process can be considered “fast” if its period of oscillation is less than three times that of the positioned valve. In such situations, the positioned valve is one of the slowest components in the loop, and therefore it slows the loop down (limits the open-loop gain of the loop and lengthens its period of oscillation). In such cases the loop without a positioner can be tuned more tightly (for higher gain and more repeats/minute); such a loop responds better without a positioner. It might also be noted that after a new steady state is reached, the positioned installation can provide more noisy control because of the hunting and limit cycling of the positioner, if it cannot keep up with the process. Some will argue that all loops can be controlled using positioned valves if the controllers are sufficiently “detuned.” This is true, but “detuning” means that the controller is made less effective (due to a reduction in the amount of proportional and integral correction), which is undesirable. Controller Algorithms On fast and noisy processes such as on flow loops, positional digital algorithms are preferred (see Section 2.4 in Chapter 2). It is also recommended that the sampling time of the flow loops should be short, because the sample period is pure dead time. Therefore, for example, if a set point change can occur faster than several sampling periods, the loop will be upset. Consequently, if large set-point changes are anticipated and fast loop response is required, one should increase the sampling frequency. It is also important that when a cascade configuration is used, the controller be provided with external reset, so that its integral mode will not wind up when its output signal is

2198

Control and Optimization of Unit Operations

TABLE 8.41f Models and Types of Ultrasonic Flowmeters by Supplier Operating Principle

Type Company

SP

CL

Automated Sonix

x

x

Caldon

x

x

x

Controlotron

x

x

x

American Sigma

IN

TT

x

x

x

Daniel

x

x x

D-Flow Durag

x

Dynasonics Eastech Badger

x

EES Elis Plzen

Fluid H

G

x

Danfoss

Datam Flutec

D

TABLE 8.41f (Continued)

L

x

x

Tokimec

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

Endress+Hauser

x

x

x

Flexim

x

x

x

Flotek UK

x

x

FMC Smith Meter

x

x

x

x

x

x

GE Panametrics

x

x

Greyline

x

Honda Instromet Kaijo

x

Kamstrup

x

Krohne

x

Laaser

x

x

x

x

Mesa Laboratories

x

x

x

x

x

Rittmeyer

x

Sick

x x

x

x

x x x x

x

x

x

x

x

x

x

x x

x

x x

x

x

x

x

x

x

x

x

x

x

D

H

G

x x

x

x

L

S

x x

x

x

x

Ultraflux

x

x

x

x

Ultrasound Res. Ctr.

x

x

x

x

Yokogawa

x

x

x

blocked from reaching the control valve, because the loop has been switched to manual. Lastly, one should keep the total loop gain constant, because the proper tuning of the controller cannot be maintained if the loop gain varies. Therefore, if the control valve gain is not constant, which is the case with all valves except the linear ones, valve compensators should be used. This is also the case with butterfly valves.

x

x

Quality Control

© 2006 by Béla Lipták

x

x

x x

Teksco USA

x

x

Polysonics

Sparling

x

x

x

x

x

x

x

Solartron Mobrey

x

x

x

Monitor Labs

Siemens

x

x

Matelco

Oval Corp.

x

x x

x

Micronics

x

x

x x

x

TT

x

x

Fuji Electric

Tokyo Keiso

IN

Fluid

SP = spoolpiece CL = clamp-on IN = insertion TT = transit time D = Doppler H = hybrid G = gas L = liquid S = steam

x

EMCO

x

x

x

x

Fluenta

CL x

x

x

SP

Thermo MeasureTech

x

x

Company

x

x

x

S

x

x

Operating Principle

Type

x x

x

x

x

Pressure Correction In the following paragraphs, three control loop configurations will be described for controlling the flow from the individual wells: 1) cascaded total flow master setting individual flow loops as its slaves, 2) combination flow/pressure cascade master, and 3) combination cascade master based on flow and pressure correction from the main collection header. Pressure correction of the flow control loop can improve the quality of flow control both in terms of accuracy and in terms of reducing the size of the upsets caused by sudden load changes. A method of pressure correction will be described in the following paragraphs that can also be useful in keeping the pressure drop across the dryers constant and in keeping the total natural gas flow from the facility constant. Different control algorithms will be described for use when the load is increasing or decreasing and to achieve bumpless transfer between the manual and automatic modes of controller operation. Figure 8.41g illustrates the response

8.41 Well-Supplied Underground Gas Storage Controls

80 75

xp(t)

70

q, x %

65 60 55 50

qt(t)

45 40

0

200

400

600

800 t s

1000 1200 1400 1600

90 85

xq(t)

80

2199

refill the underground storage system, while on the one hand using a minimum amount of compression energy and on the other hand protecting the underground geological formations from damage, caused by excessive pressure differences between the wells serving neighboring storage areas. If one based the operation only on the permeability of the “soft” wells, the MCH pressure could be relatively low and much compression energy would be saved, but the speed of recharging the “hard” wells would suffer. If the MCH pressure was maintained at a level that keeps the rate of recharging of the “hard” wells efficient, compression energy consumption would be high, and much compression energy would be burned up as pressure drops in the gas valves serving the “soft” wells. In addition, the pressure differences between soft and hard wells could cause damage to the geologic formations. The task of the control system in the reinjection mode (INC) is to find a reasonable compromise.

75 q, x %

70

Prestart-Up Settings

65 60 55 qt(t)

50

The operator manually initiates either the start-up mode (TWM1) or the operating mode (TWM2) of the control system. Prior to start-up, the operator should enter the following constants:

45 40

0

200

400

600

800 1000 1200 1400 1600 t s

FIG. 8.41g The response of the pressure-corrected natural gas flow control loop to a sudden change in set point.

of a pressure-corrected flow control system when responding to sudden change in the required flow rate.

ETMI ALSW IWFW FRSS MVSM MDPH

CONTROLLING THE INJECTION CYCLE After the winter season, the exhausted wells must be recharged. This is done by switching the system to the INC, which is shown by the dashed arrows in Figure 8.41a. During the injection cycle, the exhausted storage wells are recharged by natural gas that is lifted by compressors from other layers of the sandstone deposits of the region and from Russian gas imports. The compressors pressurize an MCH, from which the gas is distributed to the individual wells that need recharging. The pressure in the MCH is a function of the flow rate of gas delivered by the compressors and the permeability (the rate at which the spongy sandstone in the wells can absorb the gas). The wells in the more porous sandstone layers can receive and store the gas faster. These are called “soft wells.” The less porous formations can accept the gas only at a slower rate. These are called “hard wells.” The control task is to

© 2006 by Béla Lipták

DFSWi AFSW CMW MVLS MVHS

The expected total mass of the gas that is to be 3 stored, in m /h The amount of gas that can be admitted into each 3 well, in m /h The initial weighing factor for each well The slope at which the total flow rate can change, 3 in (m /h)/h The rate at which a single well’s control valve signal can change in %/h The maximum pressure difference between the w average well pressures (p in Figure 8.41h) and the pressure in the main collection header (Pc in Figure 8.41k) in bars The direction of flow that can be measurement or production A flag indicating if the particular well is active or on standby The control mode for the flow controllers on the individual wells The low-signal limit for the control signal to the valve (CV-1 in Figure 8.41h) in % The high-signal limit for the control signal to the valve (CV-1 in Figure 8.41h) in %

START-UP MODE The start-up mode (TWM1) is used: 1) during the initialization of the INC when the compressors are being started, 2)

Control and Optimization of Unit Operations

RACM CAS PID SP PV MV w

Fk From

kth

gas well

CAS FC FT LDAW MCH MMV MV N-S PC PID PT PV RACM SP

(MMV)

w

pk

P

Northern MCH

CV 1 2

Measuring MCH

2200

PT d15.75(N-S)

∆Pmax SP w k

++

LDAW for PC

d19.7(N-S) FTQ

MV

SP

PID

PV

−+

LDAW for FC Complete load distribution algorithm

Cascade Flow control Flow transmitter Load distribution algorithm for wells Main collecting header Manually manipulated variable Manipulated variable North-south Pressure control Proportional-integral-derivative Pressure transmitter Process variable Remote automatic control mode Set point

Pw

2

PC

1 —Start-up mode 2 —Normal operating mode

FIG. 8.41h The flow control loop on each well is cascaded to properly distribute the total flow and to keep the pressure drop across the control valve (CV-1) under a maximum limit.

when the flow distribution among the compressors is being changed, 3) in case of problems that necessitate that the operator initiate manual corrections, 4) prior to the activation of the safety controls to protect against the development of overpressure at the end of the injection operation (INC), and 5) during upsets that result in fast changing (high-transient) operating conditions. At the beginning of the start-up operation the control valves (CV-1 in Figure 8.41h) are totally opened and the active wells imbibe (absorb) the gas according to their natural permittivity (the rate at which gas can be absorbed). When the flow rate reaches its maximum limit, which is set for each particular well, the flow controller is activated and maintains its set point at that limit. This maximum flow stage of operation continues as long as the compressors can meet the demand by lifting the required quantity of gas. The pressure that is developed in the MCH is a function of the permittivity of the “soft” gas wells. When the compressors can no longer meet the demand for gas, the pressure in MCH drops and the remaining gas is charged to the soft permittivity gas wells. These remaining gas quantities (lent values) are delivered at flow rates that are controlled at a maximum limit for each well. During the reinjection cycle (INC), the pressure in the storage wells is increased, while the actual amount of gas

© 2006 by Béla Lipták

stored is a function of the permittivity of the individual wells. The storage pressures obtained in the individual wells will be different because of the differing permittivity of the wells, but this pressure difference must not be excessive, because the pressure differences between underground storage layers can damage the geological structures. The requirements for energy-efficient operation of the compressors is different if they are serving hard or soft permittivity wells. In case of hard permittivity wells, the compressors can efficiently charge them at maximum flow rates, but if soft permittivity wells are also served, the pressure loss across their control valves will be large, which represents a waste of energy. In case of the soft permittivity gas wells, the minimum level of compressor operation (minimum lifting energy) is insufficient to develop the pressure required for the wells to efficiently absorb (imbibe) the gas. The overall control system and equipment used during the INC of this underground gas storage facility is shown in Figure 8.41i. Operator Actions in the Start-Up Mode After the parameters that were listed in the previous paragraph have been specified, the system is placed in the start-up mode (TWM1) and is started up in manual, under the supervision of

8.41 Well-Supplied Underground Gas Storage Controls

ETMI P

CV 2

FI-2

∆ Pmax SPkw

LDAW for PC

++

d19.7 (N-S)

1

LDA 1 MFR

Pw

2 Σ FI

2

RACM CAS PID SP PV MV

1 s

Fjk

PT

PT

Pjw

FI-4

P

th From j gas well

Fi

w

Pi

w

(MMV)

Measuring MCH II

2

MCH of Southern Site

ACM Predetermined w SPi

RACM CAS PID SP MV PV

s

PT

P5

Measuring MCH I.

FI-5

1

s

FI-3 (MMV)

CV

MFR

FI-7

2

21

Predetermined 2 SPjw ACM From jth gas well

1

–+

TGQ

PT PT

FI-1

MV SP PID PV

LDAW for FC

FI-6

d15. 75 (N-S)

PT

FI-8 Own production

1

s PT

Zero-point gas (high)

From kth gas well

Pkw

Zero-point gas (low)

Fkw

(MMV)

Compressor Group I.

2

Compressor Group II.

ACM Predetermined SPkw

Measuring MCH

s

MCH of Northern Site

RACM CAS PID SP PV MV

1

MFR

2201

FI-9

P

CV

ACM CAS ETMI FC LDAW LDA INC MCH MFR MMV MV PC PID PT PV RACM SP TGQ ΣFi 1 2

Automatic control mode Cascade Expected total mass of gas stored in INC Flow control Load distribution algorithm for wells Load distribution algorithm Injection cycle Main collecting header Manual flow rate Manual manipulated variable Manipulated variable Pressure control Proportional-integral-derivative Pressure transmitter Process variable Remote automatic control mode Set point Total gas quantity Summed gas quantity The control system is in this state in the start-up mode The control system is in this state in the continous operating mode

FIG. 8.41i The equipment and control system used in the reinjection of natural gas into underground storage.

the operator. The start-up sequence involves the switching of all controllers to the manual mode and setting their output signals (manipulated variable signals to the control valves) to their low limit values (MVLS). All flow control loops on all wells (AFSWi) are activated. Compressor groups are loaded up at a rate that is less than the rate allowed (TDTR) for reaching stable state of the system. When the flows have stabilized, the operator switches the controllers to automatic and specifies the maximum total flow rate limit for the facility. The operator also adjusts the set point of the flow controller to the admissible load (AL for INC) and switches the controllers to automatic.

© 2006 by Béla Lipták

The operator checks the rate and stability of the flows into the active wells, while isolating the nonoperative gas wells. The initial flow rate that was set by the operator (AL) is overridden by the LDA, as the set points of the flow controllers are automatically modified. At this point, the local or remote automatic control mode of the flow controllers is switched to cascade control, which provides pressure correction, and the control system is switched from the start-up mode (TMW1) to the production mode (TWM2). When in the production mode, the total gas flow is distributed between the wells according to the weighing factors of the individual wells.

2202

Control and Optimization of Unit Operations

Normal Operating Mode In the normal operating mode of the injection phase (TWM2), the set point of the individual flow controllers on the individual wells is provided as shown in Figure 8.41h. The charging rate into the wells can be manually determined by the operator, can be automatically maintained as a preset portion of the total flow, or can be adjusted in a cascade manner to also consider the compressor discharge pressure and the openings of the operating control valves. The goal of the control system is to match the rate of natural gas supply generated by the compressors with the rate at which the operating well can accept that flow and to do that without either wasting compressor energy or creating unsafe overpressure conditions. Artificially determined weighing factors are used to describe the permittivity of each well. The operator can choose to switch the system into the normal mode (TWM2), if the following conditions exist: The injected total gas flow rate is stable, the starting of the compressors or the changing of their loading been completed, and the distribution of the total gas flow among the wells can be handled by making small changes in the set points of the corresponding flow controllers. In the normal mode of operation, the flow rate set points of the wells are determined by distribution of the total gas flow in proportion to WF of the wells. The flow set points are redistributed if the total injected or supplied quantity of gas changes. The change in the total flow rate has to exceed the limits of a dead zone (dead band) before automatic redistribution is initiated, but the operator can also initiate this redistribution. Pressure Correction and Flow Balancing The total flow generated by the compressors is detected by orifice plates (FI-1 in Figure 8.41i), while the flow received by the active wells w is the sum of the active ultrasonic flow sensors (F in Figures 8.41h and i). If the two flows are not the same, the imbalance will change the pressure in the MCH, and to avoid that, the total flow has to be redistributed among the active wells. Equation 8.41(1) describes how the individual flow set point is calculated for a particular well, in order to keep the pressure in the MCH (the reserve pressure in the main collection header) constant.

SP = Fi w i

min

l

−

 + 

∑ i =1

n

∑F

i

i =1

injected

 ∑ m Fi ns  SC  m i =1 injected −  ∑i =1 Fi 

 WF Fi man, aut, cas  m i  ∑ WF i =1 i 

k

∑F

man

i

i =1

8.41(1)

During reinjection (INC), constant pressure in the MCH indicates a balance between the supply and demand for gas,

© 2006 by Béla Lipták

because the gas quantity lifted by the compressors matches the amount of the injected gas mass. If more gas is lifted by the compressors than the amount of gas pressed into the wells, the pressure of the MCH will increase. When this occurs, the pressure correction cascade algorithm in Figure 8.41h will increase the set points of the flow controllers on the wells that can safely accept more gas. If the mass of gas lifted by the compressors is less than the amount pressed into the wells, the pressure in the MCH decreases and the control valves serving the soft permittivity wells will not receive enough gas, so they will fully open. When this happens, the operator will switch the system to the PCCM if the flow into the soft permittivity wells is higher than maximum allowed. In this case, the pressure correction controls (PCC) will lower the automatic and cascade set points of the flow control loops of the “soft” wells and redistribute the total flow. If the flow through the fully opened valves falls below their minimum limit for the soft permittivity gas wells, the redistribution process will stop. Because of flow sensor errors and other considerations, the total gas quantity (TGQ in Figures 8.41a and h) is calculated as follows: q2 = vq1SC

8.41(2)

where q2 is the total gas flow (TGQ) that is measured by ultrasonic flow sensors v is the average deviation (q1 − q2) of the previous sampling period q1 is the total gas flow (TGQ) that is measured by an orifice-type detector SC is a system constant In order to minimize the power consumption of the compressors, the pressure in the main collection header should be minimized. When this minimized pressure is stabilized, the flows into the active wells will differ, as soft permittivity gas wells can accept more and hard ones can take less flow. Consequently, the soft permittivity gas wells will not receive all the flow they could take, and their control valves will fully open (100%). Once a control valve is fully open, it is out of control and cannot maintain the flow set point requested by the LDA. Under these conditions a new, experience-based flow set point is applied to these soft permittivity wells as their maximum flow rate (MTSP). If the flow into a soft permittivity gas well exceeds this MTSP, the total flow is redistributed among the active wells to maintain it. The goal of this redistribution is the deactivation of the hard permittivity gas wells. The redistribution procedure modifies the set

8.41 Well-Supplied Underground Gas Storage Controls

points of the flow controllers according to the following relationship:  SPik = SPik −1 −  

n

∑ i =1

n

SPiAUT + CAS −

∑F

AUT + CAS i

i =1

 1 WF i   2 ∑in=1 WFi 8.41(3)

The redistribution procedure based on Equation 8.41(3) is executed every 15 min, until the actual flow into the soft permittivity gas well drops below MTSP. Manual, Automatic, and Cascade Control Modes The flow control loop can be in manual (MCM), local automatic (LACM), and remote cascade (RACM). When the controller is in manual, the control valve (CV-1) is manually throttled by the operator. This is the case if the ultrasonic w flow sensor (F ) is out of service. This is referred to as the manual control mode (MCM). The controller is in local automatic (LACM) at the beginning of both the start-up and the continuous operating modes or when load changes occur. In the start-up phase (TWM1), the set points of the controllers are set to a maximum value that corresponds to the admissible load of the particular well (ALSW). In the continuous operating phase (TWM2), the flow controller set point is determined on the basis of the weighing factor (WF) that is assigned to the particular well by the load distribution algorithm (LDA). This set point is also limited, so that the LDA cannot move the set point higher than the allowable maximum for the particular well. The operator can manually modify this set point when necessary. The flow controller is in cascade or remote automatic (RACM) when the pressure correction mode is active. In all control modes, the rate of change of the flow measurement signal is limited until the steady state is reached. When in the continuous operating mode (TWM2), the set point of the flow controller is adjusted to protect against overpressuring the system. Controlling Valve Pressure Drops In the reinjection mode of operation (INC), the energy consumption of the compressors rises as the pressure differential across the control valves rises. The higher this differential, the more energy is wasted. In such distribution control systems, it is recommended to use valve position-based optimization, which is described in detail in connection with Figure 8.15dd in Section 8.15. As the system is started up in the reinjection mode (TWM1), the pressure in the MCH is minimum, and therefore the valve pressure drops are also low. During this phase of operation, if the flow controller set point is limited to its maximum flow rate, the differential pressure between that of the MCH and the average pressure at the side of the gas wells will not be excessive.

© 2006 by Béla Lipták

2203

During the continuous mode of operation (TWM2) the pressure drop across the control valves is limited to a programmed value (dPmax). The development of an overpressure in the MCH can be eliminated by sending more gas to the best permittivity gas wells. This cascade controller configuration for the best permittivity wells is shown in the lower part of Figure 8.41h. The set point of these controllers can be calculated by the following equation: CAS

SPi

FLOW

= SPi

× MVmax

PRES

+ MV

MAX

(Fi

PRESS

FLOW

– SPi

) 8.41(4)

The cascade controller shown in Figure 8.41h serves to lower the pressure in the MCH by increasing the set points of the flow controllers serving the high permeability wells, according to Equation 8.41(4). This nonlinear algorithm stabilizes the MCH pressure at a level that is below the allowable maximum. These algorithms can also respond to fast pressure transients, because they do not contain any ramp functions that would limit the rate of rise of their measurements. Protecting the Compressors If the discharge pressure or the rate of change of the discharge pressure of any compressor exceeds its predetermined limit, the control system is automatically switched back into its start-up mode (TWM1). The system is also switched automatically to the start-up mode if the control valves on half of the operating wells exceed their 90% opening. Alarm signals are actuated if the tale gas flow of the gas well that is under pressure-corrected cascade control reaches its maximum limit. Another condition that initiates a warning signal is when the flow rate to half of the active wells exceeds their predetermined maximum admissible load settings (MVAL). Another element in the safety logic of this control system is that if the opening of a valve exceeds 80%, it cannot be switched to cascade. In that case, the operator has the option of either activating new wells or switching controls of that well to the start-up mode (TWM1). Additional safety-related logic steps include the stopping of compressors if, after activating the start-up mode (TWM1), the total lifted gas quantity (TGQ) is less than 25%. After the compressors are returned to stable operation at their expected OPs, the system can be returned to the operating mode (TWM2). The control system will automatically switch to the start-up mode (TWM1) if the discharge pressure or its rate of rise exceeds a preadjusted maximum limit value.

CONTROLLING PRODUCTION CYCLE The natural gas that was compressed into the underground storage facility during the summer months is used up in the winter. Therefore, as the heating season approaches, the gas wells are switched from the injection (INC) to the production

2204

Load

Control and Optimization of Unit Operations

SPid 2 1

SP PID Flow MV controllers for PV dryers1...3

PT

Fid

D-01-1 ... D-01-3

2 MV

PV

1 From overpressure protection system

P3 F

P Override 1...3 less signal

MPFRVD

Load

SPid

2 1

M C H

P

SP PID Flow MV controllers of PV dryers 4...8

MPFRVD

Main collection header Maximum predetermined flow rate through dryer The control system is in this state in the start-up-mode The control system is in this state in the continuous operating mode

D-01-4 ... D-01-8

F

Pc

P

Override 4...8 less signal

CV 2

(a)

MCH MPFRVD 1 2

SP for pressure SP of MCH PID

From over-pressure protection system

Fi d

P CV 2 (b)

FIG. 8.41j Dryer flow control algorithms for low- and high-capacity dryers. Note that the high-capacity dryers (b on the right of the figure), when in their start-up mode, are provided with a pressure override, which controls the pressure in the main collection header (MCH).

(PRC) mode. In this mode of operation the gas that is stored at about 1450 psig (100 bars) in the wells is admitted under flow control into the main collection header (MCH in Figures 8.41a, j, and k). This header is held at 1087 psig (75 bars) and supplies the large- and small-capacity dryers (Figure 8.41j). The gas flow through the individual dryers is under flow control. The set points of the individual flow controllers (FIC-2 in Figure 8.41a) are adjusted so that, on the one hand, their total flow matches the total demand of the users. On the other hand, these set points are corrected to keep the pressure in the MCH (PT-3 in Figure 8.41a) at 1087 psig, while keeping the “zero point” pressure (the pressure at the beginning of the distribution header, detected by PT-4 in Figure 8.41a) below its high limit of 870 psig (60 bars). Start-Up After the START command, the flow controllers on the individual gas wells receive their set points on the basis of the LDA and move to that point at a preset rate (FRS). After the pressure of the MCH has stabilized, the required (expected) total gas flow (ETMP) is sent through the dryers in a distribution set by the operator. In the start-up mode (TWM1) of the production cycle (PRC), the flow control loops on each active well are switched to their local automatic modes (LACM), and their set points are automatically established according to Equation 8.41(5) as follows:

(

prescribed SPiw = Qtotal −

© 2006 by Béla Lipták

WF ∑ Q ) ∑ WF manual i

i

n i =1

i

8.41(5)

where w

SPi is the set point of the flow controller on the individual well WFi is the weighing factor for each individual well, which determines its share of the total flow required The set points of the flow controllers are gradually increased according to the rate determined by a preprogrammed length of transient time (TDTR), which sets the new value of this set point, determining its slope. Similarly, the task of the control valves downstream of the dryers (CV-2 in Figures 8.41a, j, and k) is to maintain the pressure in the main collection header at a pressure of 1087 psig (75 bars). This is necessary in order to provide a steady pressure drop across the dryers. At the same time, the flow rate through the individual dryers is set to its predetermined maximum flow rate, and override controls are actuated if that limit is exceeded. Pressure Correction During the production cycle, it is necessary to apply pressure correction to the MCH in order to maintain a constant pressure drop across the dryers, but this goal is subordinated to the necessity of meeting the total user’s demand for conditioned gas. The gas flow from the individual wells will be constant if the pressure in the MCH is stable, because this keeps the pressure drop across the well flow control valves (CV-1 in Figures 8.41a and j) constant. It is desirable to keep the MCH pressure relatively high, because that provides the driving force for high production rate. This relatively high pressure can be maintained as long as the pressure in all the active gas wells is greater than the pressure in the MCH.

8.41 Well-Supplied Underground Gas Storage Controls

Continuous Operation After start-up, the system is switched into continuous operation (TWM2), during which the supply from this UGS must match the gas demand of the countrywide distribution network. The actual total gas flow (TGQ) obtained during startup (TWM1) is usually not equal to the expected total (EFRD). In order to correctly match the supply to the demand and, thereby, stabilize the pressure in the main collection header, the control system has to be switched into its continuous operating mode (TWM2). Before changeover, the operator must set the allowable set-point ramp rate for the dryers (SPR) and the admissible or required load or production rate for each dryer (AL). The pressure controllers (PCc in Figure 8.41a) that control the pressure in the MCH are configured as the cascade masters of the well flow controllers that are assigned to pressure correction. Also before changeover to the continuous production mode (TWM2), the individual well flow controllers must be in automatic and should be delivering the gas flows matching their set points. See Figure 8.41j for a schematic description of the two algorithms used for dryer flow controllers. In the continuous mode of operation (TWM2), the system automatically performs the following functions: It calculates the total gas flow that the UGS should produce (ETMP). The operator then sets the set points of the dryer flow controllers based on this total required flow, using the LDA to make sure that each dryer is assigned the right flow. When the load demand is changing, the algorithm revises its recommended set points, which are then modified by the operator. In determining the set points for the individual dryer flow controllers, the control system limits that value to the maximum flow rate that is allowable through the particular dryer (MFRD). If the calculated set point is below the MFRD of the dryer, the operator is not allowed to modify that setting. The dryer flow set point is calculated as follows:  SPid =  ETMP − 



n

∑ MPFD  ∑ i

i =1

MVAL i MVAL i

n i =1

8.41(6)

where d

SPi is the set point for the particular dryer ETMP is the required (expected) total amount of gas to be produced MPFDi is the predetermined maximum flow rate that the particular dryer can handle MVALi is the maximum value of the flow that a particular well can produce If some of the dryers are already operating at their maximum (MPFD), the required total production (ETMP) must be met by setting the flow controllers on the other dryers, in accordance with Equation 8.41(6).

© 2006 by Béla Lipták

2205

Switching the Operating Modes When the operation is stable, the operator can switch the system back into the startup mode (TWM1), but the normal mode of operation is continuous (TWM2). When the system is in the continuous mode, the set point of the flow controllers on the individual wells is cascaded for pressure correction (PCCM), while the flow controllers on the dryers are in their automatic flow control mode. Switching between operating modes should be bumpless, and the pressure in the MCH should be controlled by pressure correction (PCCM). The set points of the dryer flow controllers remain the same as existed before the mode change. If the operator changes the set-point values, the rate of change is limited by the slope that is predetermined for each dryer (FRSD). Control Modes and Loops Figure 8.41k describes the equipment and controls used in the production mode of operation. The main components of this system include the wells that supply the gas; the MCH, which collects and transports the gas to the dryers; and the header, which takes the gas to the users. The demand for the gas is determined by the users, while the supply is a function of the gas flows from the operating wells. If the supply and demand are out of balance, the pressure in the MCH will change. The control system described in Figure 8.41k serves to match the supply to the demand, while protecting the component subsystems (wells, dryers) from sudden upsets or from overpressure conditions. Because both the accuracy and the rangeability of the flow sensors are limited, pressure correction is used to maintain the balance between gas supply and demand. Controlling the Wells If the ultrasonic flowmeter that is serving the flow control loop of a particular well fails, the loop is switched to manual. Under this condition, the flow is estimated on the basis of control valve pressure drop and opening. The well flow control valve (CV-1 in Figure 8.41a and k) is switched to the local automatic mode (LACM) whenever sudden upsets are experienced or when the operation mode is being switched between the start-up (TWM1) and continuous (TWM2) operating modes of operation. In this case, the flow controller set point is provided by the LDA, but can be modified by the operator. When the set point is being changed, the rate of change is limited. Minimum and maximum flow limits are also provided for each well. When the well is in continuous operation (TWM2), pressure correction is applied to the flow controller’s set point (PCCM). If a controller is in its remote automatic mode (RACM), the operator is not allowed to modify its set point.

2206

Control and Optimization of Unit Operations

EFRVD ETMP LACM LDAD LDAW MPFRVD 1 2

Expected flow rate value through dryer Expected total mass produced Local automatic control mode Load distribution algorithm for dryers Load distribution algorithm for wells Maximun predetermined flow rate value through the dryer The control system is in this state in the start-up mode The control system is in this state in the continuous operarting mode Users 1 870 PSI 2

PID

1087 PSI set by the operator

In TWM2

P3

TWM2

PV

LDAD

1

1

PID MV PV

EFRVD (set by the operator) SPjd

LDAW for calculating SP' of single gas well flow controllers

2

SP

PID MV

s

TWM1

In TWM1

SP

ETMP

SP

MV Overpressure PV controller

P4

2

2 PID Dryer MV controllers PV 4 ... 8 SP

SPid

MPFRVDj

1

2 PID Dryer MV controllers PV 1 ... 3 SP

SPiw

s PERi Fiw

From ith gas well

LACM PERj

th

From j gas well

s

Fjw

SET PID SP Single flow MV PV controller of gas well Pw i

P

P

F

Override controller 1...3 less signal

CV 1

SET PID MV SP Single flow PV controller of gas well

D-01–1 ... D-01–3

D-01–4 ... D-01–8

w

Pj

1

MCH

LACM

MPFRVDi

Fid

P

F Pc P Override controller 4...8 less signal

From ith dryer toward compressors and zero-point

CV 2 Fjd

P

From jth dryer toward compressors and zero-point

CV 2

P

Users

CV 1

FIG. 8.41k The overall control system of an underground gas storage facility, operating in its continuous-production mode (PRC).

Dryer Controls As shown on the left side of Figure 8.41j, the three small-capacity dryers are under flow control during start-up (TWM1). In this mode, the operator adjusts the dryer flow controller set point, using the LDA as a reference. When the total flow demanded by the users changes, the set points of the active dryer controllers are recalculated, based on the predetermined flow rate of the total system (PFRU). In addition to flow control, the dryer control loops are also provided with overpressure protection, as shown on the left side of Figure 8.41j. The right side of Figure 8.41j illustrates the controls used on the five high-capacity dryers. In the start-up mode (TWM1), their controls are in the PCCM, but their set point cannot exceed the maximum flow (MPFD) set by the operator. In the continuous mode of operation (TWM2), the operation of these high-capacity dryers is similar to that of the small-capacity dryers. Therefore, they are operated in the PCCM in order to

© 2006 by Béla Lipták

maintain the pressure in the MCH at the setting specified by the operator.

Pressure Correction In the start-up mode (TWM1), the set points of the control loops on the high-capacity dryers have to satisfy several considerations. They have to be under the maximum flow allowed for the particular dryer. In addition, they have to be pressure corrected. The pressure correction considers the pressure in the MCH (P3 in the figures) and the pressure at the start of the supply header serving the users, which point is also called the “zero point” (P4 in the figures). In the continuous operating mode (TWM2), the flow controllers of the active wells develop a relatively steady pressure in the MCH. The FICs on the wells are in their RACMs, and their set points are adjusted by the operator. If the control valves (CV-1 in the figures) on half of the active

8.41 Well-Supplied Underground Gas Storage Controls

the wells are more than 80% open, the operator is warned to lower the pressure in the MCH by reducing the set points of the FICs on the wells. Warning the Operator Usually, the load redistribution is recommended to be done per Equation 8.41(3). When this is not feasible, the control system advises the operator if the flow set point from a particular well is no longer available for pressure correction. The operator is also advised by the control system if the maximum or minimum allowable flow limit is reached for a particular well. The control system also warns the operator if any well control valve has opened beyond 80% or if the flow through them is outside allowable limits. When the demand for gas cannot otherwise be met, the operator must activate new, previously passive wells. Normally, the control system keeps the total flow from the wells that are under pressure-corrected control within the minimum and maximum tale flow limits (mCC and MCC). If the flow from the wells that are controlled in the PCCM reaches the preset limit for maximum tale flow (MCC), the control system must warn the operator. The control system also warns the operator if any well control loop with a pressure-corrected set point (PCCM) opens its control valve to more than 80%. Controlling the Pressure at “Zero Point” The “zero-point” pressure is the pressure at the beginning of the header that serves the users (P4 in Figures 8.41a and k). This supply pressure to the users is not controlled directly, as it is the consequence of the balance between the gas supply and demand. Therefore, it is allowed to float between 725 and 870 psig (50 to 60 bars). At the beginning of the PRC, the outlet pressure of the active wells is about 1450 psig (100 bars), and under such conditions, overpressure conditions can arise. As shown in Figure 8.41k, the overpressure protection control loop is set at 870 psig (60 bars), and if the zero-point pressure reaches that level, the controller will start to throttle down the dryer flow control valves (CV-2). A high-pressure alarm is also provided. Just as the pressure in the main collection header is a function of the balance between the total flows of the wells and the dryers, the zero-point pressure is an indication of the balance between the flow supplied by the dryers and the flow demanded by the users. In more sophisticated control systems, feedforward control can be used on the basis of the gas demand, so that the system does not wait until a pressure upset is caused by the supply–demand imbalance, but eliminates it as soon as it occurs. THE TOTAL CONTROL SYSTEM The control system is so designed that it has to be switched into the start-up mode whenever the OP for the overall facility is changed or when upsets occur. In that mode (TWM1), the

© 2006 by Béla Lipták

2207

individual wells are on straight flow control. The LDA is used to distribute the total gas production (ETMI), according to the WFs of the individual wells. Because of the limited reliability of the flow sensors under changing flow conditions, the total gas flow obtained from the wells (ETMI) is not accurately known during startup. Once the flows have stabilized, the system is switched into the continuous operating mode (TWM2). In this mode, when the operating point for the whole system (OP) changes, the pressure of the main collection header (MCH) reflects that change, and the automatic pressure correction (PCCM) adjusts the flow set points at the individual wells. As shown in Figures 8.41j and k, the control system is reconfigured as the operating mode is switched from startup (TWM1) to continuous (TWM2). In the figures these modes are indicated by the numbers 1 and 2. Existing DCS Hardware The control hardware consists of two fiber-optic bus-connected DCS units. The first DCS contains the controls for the well side of the total UGS, ten field control stations (FCS), four human interface stations (HIS), three operator consoles, one PLC serving data acquisition, another PLC for safety controls, and a number of ultrasonic flowmeters. The second DCS controls the gas conditioning and drying process. It is provided with two field and two operator consoles. All the FCSs, including their controllers, digital and analog I/O, CPU, power, and communication boards, are redundant. The HISs consist of an industrial computer with hard disc, 21 in. monitor, trackball, and keyboard.

CONCLUSIONS AND COMMENTS The goal of the existing control system is to accurately meet the demand for natural gas of the users while minimizing upsets or disturbances. This task is made more difficult by the changing conditions in the wells during both charging (INC) and discharging (PRC) of the wells. An added source of difficulty is the low accuracy and unreliability of the flow sensors. In addition, the capacity and safety limitations of both the wells and the dryers cannot be violated and is desired to minimize the energy cost and maintenance of the operation. One of the tools used to achieve these goals is to apply pressure correction to the flow controllers at both the wells and the dryers. Another tool is to use the pressure drop ratio (∆p/p1) and the control valve openings to estimate the flows passing through them. The reader might notice that the control system described in this section is one that probably evolved from a manually operated facility; it is still operating with unreliable sensors when water is present in the gas and control valves that have no direct stroke position detectors. Under these conditions, it

2208

Control and Optimization of Unit Operations

Set at 90% integral only FB DA > VPC

EFB RA KWIC 03 EFB

SP >

SIC