Pipeline & Gas Journal
February issue 2000:

Salt Cavern Withdrawal

Revamped Control System
Fixes Severe Noise, Vibration Problems

by Terry Carrington, Supervisor, Salt Cavern Storage,
ATCO Pipelines, Edmonton, Alberta, Canada;
Thomas H. Ramsay, RAMCO Energy Products Ltd., Calgary, Alberta; and
Laurence R. Stratton, Manager, Technical Services, CCI,
Rancho Santa Margarita, California

ATCO Pipelines operates six natural gas caverns in the Fort Saskatchewan area of Alberta. Five caverns were developed in 1982 with the addition of Cavern #6 in 1994. Total system deliverability is critical because the facility functions as a peak shaver for gas supply to several petrochemical and power plants along the North Saskatchewan River.

The pressure letdown valves for all six caverns are housed in the Injection/Withdrawal Building and share a common control system. Gas withdrawals from each cavern are controlled to ensure cavern pressures do not drop below 8 MPa (1,150 psi) to prevent excessive creep and possible cavern collapse.

Gas-withdrawal control valves and signals for the original five caverns were configured as shown in Figure 1. Each run was originally designed to flow 2.8 million m3/day (100 million SCFD).
Two cavern pressure ranges define the control valve system operation – High Pressure (HP) operation above 14 MPa (2,000-psi) and Low Pressure (LP) operation below 14 MPa. In the accompanying table, the valve flow coefficient (Cv) describes each valve’s design characteristics:

HP LP
6-in. monitor globe Open Modulates
control valve (Cv=105)

4-in. working globe Modulates Open
control valve (Cv=87)

6-in. ball valve Closed Open
(Cv=790)

In HP operation, the 100-mm (4-inch) control valves created extremely high noise and vibrated excessively. In fact, there was a very real safety consideration and some valve damage had occurred. As a result, all five trains were limited to a maximum withdrawal rate of 2.5 million m3/day (88 million SCFD) in actual operation.

New Withdrawal System Design
In 1994, the sixth cavern was added, and the original control valves from the #2 Cavern, Figure 1, were moved to become the new #6 Cavern withdrawal system. A redesigned #2 Cavern control system, Figure 2, was installed to include a 6-inch (150-mm) multi-stage, pressure reduction valve which was set up to control both HP and LP modes.

In the new system, the previous 4-inch working control valve and the 6-inch ball valve have been removed and replaced by one 6-inch working control valve as shown in Figure 2. Also, the trim in the old 6-inch monitor valve has been replaced to increase its Cv. System operation has been modified to:

HP LP
6-in. monitor valve Open Open
(Cv = 200)

6-in. working valve Modulates Modulates
(Cv = 218)

The radically designed system is now in successful operation on Cavern #2, the largest of the six storage caverns. In addition to solving the vibration and noise problem, a significant benefit of the new arrangement was the 70 percent increase in the capacity rating from the derated flow capacity of 2.5 million m3/day (88 million SCFD) to 4.25 million m3/day (150 million SCFD) for Cavern #2. The new design eliminated the need for control-valve switching since the monitor valve does not modulate in either HP or LP modes. It is only used as an emergency backup valve.
Hence if Cavern #2 were directly controlled in parallel with the other trains it could, in effect, draw nearly twice the gas from this cavern as from the others in parallel with it. It therefore became necessary to essentially “fool” the control system into thinking that it could be operating in single or dual-cavern mode.

A special signal conditioner interposed between the incoming common signal and the new working control-valve actuator in combination with the customization of the new system valve Cv characteristic to match the performance of the other cavern withdrawal system runs in the HP, LP and dual modes.

New Working Control Valve
The new working control valve as installed, Figure 3, (sectional elevation, Figure 4) eliminates the severe noise and vibration problems through velocity and kinetic-energy control within the valve trim at any flow. The multi-stage, pressure-reduction trim in this valve consists of a stack of tortuous-path disks, Figure 5, having a multiplicity of right angle turns. By forcing the gas to flow through these paths, the kinetic energy in terms of velocity head (Pvh=pV2/2) is controlled to below 480 KPa equalizing ring (PER) around the inside diameter of each disk in the stack assures that equal pressures act radially around the circumference of the plug in any position in its stroke. This design keeps the plug centered at all loads and prevents plug vibration.

Also, this disk stack is “characterized,” i.e., discrete groups of disks in the stack are designed for different pressure drops by incorporating a different number of right-angle turns. Figure 6, “System Capacity, Cv vs. Signal Percent To Modulating Valve,” shows the effect of this characterization on the new system capacity. This characterization, together with the signal conditioning previously mentioned, permits this Cavern #2 gas-withdrawal system to operate as needed in four different modes – (1) HP simulating one train, (2) HP simulating two trains, (3) LP simulating one train, and (4) LP simulating two trains. In all cases, a scaling factor is applied which means that the signal conditioner is, in effect, telling the valve actuator to open only by the scaling factor shown in relation to full open in each case when the common input signal calls for full open. Of course, this scaling factor applies proportionately throughout its full flow control range.
To simulate a single, high-pressure gas-extraction train consisting of a modulating 4-inch working valve (Cv = 87) in series with a wide-open 6-inch monitor valve with reduced trim valve (Cv = 105), the signal to the new 6-inch control valve requires a 0.65 scaling factor. This will adjust the travel of the new 6-inch control valve to 65 percent of its total travel and the system capacity to a Cv of 70.
To simulate two parallel, high-pressure, gas-extraction trains each consisting of a modulating 4-inch working valve (Cv = 87) in series with a wide-open monitor valve (Cv = 105) the signal to the new control valve requires an 0.96 scaling factor. This will limit the travel of the new control valve to 96 percent of its total travel and the new system capacity to Cv = 140.

To simulate a single, low-pressure, gas-extraction train consisting of a modulating 6-inch monitor valve (Cv = 105) in series with a wide-open 4-inch working valve (Cv = 87) and a 6-inch ball valve (Cv = 790), the signal to the new control valve requires an 0.82 scaling factor. This will limit the travel of the new control valve to 82 percent of its total travel and the new system capacity to a Cv of 110.

To simulate two parallel, low-pressure, gas-extraction trains each consisting of a modulating 6-inch monitor valve (Cv= 105) in series with a wide-open 4-inch working valve (Cv = 87) and a 6-inch ball valve (Cv = 790), the signal to the new control valve is not scaled. The two parallel lines would have a total capacity Cv of 210. The new control-valve system provides a system capacity Cv of 145. Wide open, the new control-valve system valve Cv is large enough to pass 4.25 million m3/day (150 million SCFD) with only a 1.8-Mpa (260-psi) drop across the system.

Figure 7, “The System Cv vs. Cavern Pressure” chart shows the system capacity required to flow 4.25 million m3/day (150 million SCFD) at a given inlet pressure. The curve assumed an outlet pressure of 11 MPa at a given inlet pressure. The curve assumes an outlet pressure of 11 MPa (1600-psi). The chart shows that the high-pressure drop flows are handled at low system capacities. To keep these pressure drops from causing vibration, erosion and noise, the working control-valve trim is designed with more pressure protection at low Cv conditions.

Figure 6 also shows the total system capacity through the monitor and new control valves as a function of full control-valve stroke. The valve provides 20 stages of pressure reduction at a Cv of 18; 16 and 20 stages to a Cv of 34; 12, 16 and 20 stages to a Cv of 77; 6, 12, 16, and 20 stages to a Cv of 98; and 2, 6, 12, 16, and 20 to the full system Cv.

Ever since the startup of the new pressure-control let-down system, #2 Cavern gas withdrawal run has experienced minimal noise/vibration and no control-system damage. During periods of high demand, this cavern can be counted on to deliver up to 70 percent more gas than any of the other caverns with greater rangeability and without the need for run switching. P&GJ