Custody Transfer Meter Skid Back Pressure Control Valve Issues 2

[Tom Connor]

In 40years of experience I've never considered (or seen) a CTM back pressure control valve used to mitigate a surge event. My question, is there anyone out there that have or would consider using a CTM back pressure control valve to mitigate s surge. I've real issues with this and question the validity of doing so.

Tom Connor, PE
Operational Fluid Mechanics

 

[hacksaw]

Agreed there, surge can mean many things, here it appears to be referring to "water hammer" or hydraulic shock associated with rapid valve closure as you have suggested, but the topic does seem to have evolved...

 

[1503-44]

Agree totally with LittleInch, if there is a slam shut valve at the tank, they can be very problematic. Everything happening there reflects back to hit the meters. I would question the need for such quick action downstream of the meters at an onshore tank farm. Tanks should have hi & hi hi level alarms and sufficient freeboard above those so as to not make them necessary for overflow control, so what makes a snap acting valve necessary there? The tanks not shoving off a bouy, or going anywhere else and they are usually just watched closely and let to burn themselves out over several days even if they catch fire. A couple hundred barrels more in the tank wont make much difference. Slow those down. Snap acting ESD valves should be located upstream of the meters and PCV unit where there is more HP pipe upstream to contain full surge pressure. Put the surge control skid (if needed) inbetween the ESD and a HP PCV, which tends to keep most surge pressures directed upstream and/or moving into the surge skid as it closes, then the meters and lastly the backpressure controller. Even so you still may need the surge skid. I have designed many pretty high flow product and crude pipelines and that arrangement always works, sometimes without a surge skid, and even with a snap valve at the tank, but I did manage to keep station velocities lower than you did.

 

[georgeverghese]

Flowing velocity would be much less than 14fps in the last 10-20% of the closing stroke of the tank MOV, since the upstream 600lb/150lb PCV would have throttled back to maintain set point.

 

[Tom Connor]

In this system the Tank Valves are Gate valves with a 2.5 minute closing, flow does not reduce until the last 5-10% of stroke so the PCV upstream of the meter can solve this without issue as long as the CTM check valve remains open to the pressure wave. In the manifold 12 butterfly valve close in 1 minute and flow is being reduced during the entire stoke, this requires the PCV upstream of the meter to close before the butterfly valve completely closes. The issue lies in a failure of the CTM PCV, the valve is "fail in place" however situations have occurred where the valve failed and did not "fail in place" but actually closed completely driving the upstream pressure over MASP because the CTM PCV is faster than the PCV upstream. Recommended solution is to close the upstream PCV faster than the CTM PCV. Client is strongly considering "mechanical stops" on the CTM PCV, which we do not approve of simply because the CTM is not designed to take the unbalanced forces across the closed CTM PCV. Best solution is to increase the closing time of the upstream PCV.

Tom Connor, PE
Operational Fluid Mechanics

 

[LittleInch]

Well that's a different issue altogether.

Can you slow the CTM cv valve down? Why is it so fast acting?

In the case you describe above I can now understand why this action is being proposed as it's a hopefully rare event but has potential for severe consequences.

What are these "unbalanced forces"?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

It is still not clear why this CTM "backpressure valve" is acting the way you describe it:
a) If it is an automated backpressure CV holding back a min backpressure for the CTM, it ought to go open as the downstream tank MOV closes
b) Why is this CTM backpressure valve fail in place? Most automated backpressure valves fail close.
What kind of control valve is this?

 

[Tom Connor]

I absolutely agree with the CTM response as it should go open (valve is a fisher V150), however the client instructed the automation team to close the valve on any downstream valve movement since the spec break PCV valve is slower that the CTM PCV and the CTM is much closer to the nearest downstream MOV that would be closing. Were not sure the reason yet but we believe the actuator on the spec break PCV may be limited in capability. Client has yet to proved the details on the actuator. We've recommended making the spec break PCV valve the fasted valve in the system. As I've stated were dead set on not recommending/approving mechanical stops.

Tom Connor, PE
Operational Fluid Mechanics

 

[georgeverghese]

Okay. Looks like there is room for improvement on the control response of this upstream 600lb PCV - a local instrument air volume booster may help.
Think there may be more to this check valve at the CTM - these are the typically the cause for piping water hammer for pump sets in parallel. Swing checks are known to chatter at low flows - see if a damped check valve will help ( Norris, Mokveld). This check valve may transiently go close when switching from one tank to another, when the tanks are at different levels.
Presumably there is a blocked flow PSV downstream of the 600lb PCV, supplemented by a high pressure trip loop.

 

[1503-44]

"to close the valve on any downstream valve movement". What?

I'm convinced this is a control logic problem far more than any configurational issue.

 

[georgeverghese]

Agreed, given there are now 2 control loops on this imcompressible liquid line ( 600lb letdown loop and fisher V150 backpressure control loop ) in series, one can expect control instability due to controller interaction. A suitably gas filled bladder or similar between these 2 control valves may help to soften things and improve pressure control stability (as you suggested earlier).
Ideally, the CTM skid should be a 600lb system, with only one fail close PCV in this line. But given the current circumstances,
(a)if control stability can be improved at the 600lb PCV
(b) the MOV slowed down adequately
(c) the CTM backpressure CV be made to FO or fitted with mechanical stop to prevent surge
(d) the check valve be switched out to a damped dual plate check or similar (else move the check valve to be right next to the tank MOV).
(e) a full flow blocked in PSV is installed downstream of the 600lb PCV, or at the very least, a 2oo3 high pressure trip loop with SIL 2 SDV
(f) optionally, reduce pumping pressure upstream of the 600lb PCV when any tank MOV begins closing
then I dont see this otherwise awkward design as being completely unacceptable.

 

[georgeverghese]

There is a basic process controls flaw here: there are 2 PICs' trying to maintain set pressure on the segment between the 600lb PCV and the CTM bacpressure CV. Control instability can be expected no matter which way you wish to operate these controllers in the current setup. This has to be corrected first before attempting to address liquid hammer issues. The CTM skid should be a 600lb piping system.

 

[Tom Connor]

Gentlemen, all of the comments have been greatly appreciated and excellent comments for which I've thoroughly agreed with most all the made. Meter skids have long been purchased by our client, are in process of being installed and I've not agreed to seal the design. The solution that we will accept involves no limit stops, increasing the actuator speed of closure on the facility incoming PCV and decreasing the speed of the CTM PCV's. We've dynamically proven that the distance between the PCV's will be stable within the automation control loops without control instability. When necessary we agree to address any minor surge issues with nitrogen filled bladder tanks. With proper PCV speed controls between the two PCV's there was only one location that required a bladder tank less than 150 gallons.
Thanks for all the comments and suggestions.

Tom Connor, PE
Operational Fluid Mechanics