using pressure control valve as blowdown valve

[dungun]

Hi
I need some opinion on the blowdown system for liquid petroleum gas storage. Some designer do not used any BD mechanisme except the PSV. However if I used PCV as BDV for blowdown any restriction from API 521?. or other codes?. Please

 

[JLSeagull]

The PSV is not my choice for the blowdown valve. In low pressure classes such as 150 or 300 consider a shutdown valve with an orifice plate to control the release rate. In class 600 or higher consider a shutdown valve and choke to take the drop. I normally use an API 6D ball valve for the shutdown valve.

I think that API requirements for the blowdown duration exist in the API 14 series offshore standards. However I did not find it in a quick word search. Perhaps they use depressure or something else in the standards.

 

[dungun]

Hi JL

I means that the Pressure Control Valves used as Blowdown valve (depressurized). How??

Regards

 

[slacey54]

I'm no engineer but 30 years experience. PSV's are PSV's. They are not intented to be used for blow down valves. PSV valves are normally very expensive and have an almost zero bubble shut off. So, if used for more than a PSV, and lets say it leaks for any reason. If there is no other PSV that will relieve the operation the process will have to be shut down until the leading PSV is repaired. In addition there is no way to do a controlled blow down (restricted flow for whatever reason) with a PSV. PSV's are designed to open fully when reaching their set pressure.

 

[TD2K]

Modulating pilot operated PSVs do not go fully open when the set pressure is reached. Rather, the pilot adjusts the pressure on top of the dome to maintain, essentially, the inlet pressure at set pressure, opening and closing the disk as needed. In effect, they act akin to a control valve.

Anderson Greenwood has a feature that will allow you to dump the pressure on top of the dome remotely causing the PSV to go fully open which would allow the PSV to function effectively as a blowdown valve (they call it a 'remote unloader'). However, blow down valves are usually quite a bit smaller than PSVs in my experience as they aren't typically sized for the same flow rates. A PSV could be sized for a large vapor flow rate as a result of a fire or gas blowby while a depressurizing valve is typically sized for the gas inventory in the unit that needs to be vented as a result of an ESD (emergency shut-down). Common requirements that I've seen for a blowdown valve is to depressure a unit to 100 psig (or 1/2 of design pressure, whichever is lowest) in 15 minutes.

Remember if you use the PSV as a depessurizing valve to consider low temperatures in the system if you have any liquids that will autorefrigerate. That comment also applies to a depressuring valve.

 

[dungun]

hello friend
Agree that the PSV cant be used as BDV, but how about the PCV used for BDV?..

 

[djack77494]

I don't see how you'd use a PCV as a blowdown valve. Acting as a backpressure device, the PCV would be designed to maintain pressure in the system. It would maintain the normal pressure and any higher pressure that were still below the point where you'd want to move into isolate and blowdown mode. If this is true, the setpoint would be up high - at the point where you're running into trouble. When the PCV starts to open and bleed off pressure, it will soon close. It will not lead to a low pressure condition, which is what I assume you want to achieve. Perhaps you'd better tell us by what you mean by blowdown.
Doug

 

[JimCasey]

I agree with Doug if the PCV is a self-contained backpressure regulator. Even then, it may be overridden with a solenoid valve on the (external) sensing line.

But if the PCV is a "CONTROL VALVE" it will follow any signal sent to it.
Current advanced state of the art control systems accomplish this easily. Just program in a function block to dump the valve open. Slightly more primitive systems may be more simply adapted by inserting a 3-way normally closed solenoid valve between the positioner and the valve actuator. With a fail-open (Air-to-close) control valve, chopping the power to the solenoid will isolate and vent the actuator, blowing the control valve to full open.

Blowdown can be pretty harsh service. High velocity, entrained particles are pretty tough on the valve trim and as cited in an earlier post, the valve may not shut off as tightly as before the blowdown. I remember once seeing a globe valve plug from a digester vent valve-which is not supposed to be as harsha service as blowdown, and it had suffered easily 50% material loss. Moral of this story: Use hardened trim, oversize the actiator for more seating load.

THis is an application where a ball valve can serve well. Use a full-port valve and get it open as quickly as possible. Then the high-velocity flow will not churn around inside the body doing damage, as it can with a globe-style valve with its S-shaped flow path.