Relief Valve is causing hammering
Relief Valve is causing hammering
(OP)
Dear All,
We have a 6" PSV (Set 55 KG/cm2, @ 220 Deg. C) Service is Ethylene glycol + water. The relief discharge is injected to a closing system 25 m away from relief point. We encountered a hammering when this PSV opens. An Idea was raised to elongate the relief line to about 60 m long and injected to a very low pressure line.
Can you please help to solve this problem. What is the best proffesional check list i must confirm to know what cause this problem.
Regards
SHARQ
We have a 6" PSV (Set 55 KG/cm2, @ 220 Deg. C) Service is Ethylene glycol + water. The relief discharge is injected to a closing system 25 m away from relief point. We encountered a hammering when this PSV opens. An Idea was raised to elongate the relief line to about 60 m long and injected to a very low pressure line.
Can you please help to solve this problem. What is the best proffesional check list i must confirm to know what cause this problem.
Regards
SHARQ





RE: Relief Valve is causing hammering
Is that what's happening?
Good luck,
Latexman
RE: Relief Valve is causing hammering
Hammering because of hot reliefed service is being injected to a cold service. The PSV setting is OK and we do not encounter any problem in PSV setting nor functionality. We encounter only a high vibration caused by hammering.
Regards
SHARQ
RE: Relief Valve is causing hammering
Some energy storage in the line is necessary to prevent extremely fast pressure variationsj, both up and down.
Latexman is correct, if there is some but not much energy storage in the line, then a much smaller valve may help. An accumulator of some sort can also help, or if you already have one, is it on and working?
PUMPDESIGNER
RE: Relief Valve is causing hammering
It is not vented, The service is very toxic (Contains Ethylene Oxide). The discharge of PSV connected to a 6" pipe that transfer the reliefed product to a bigger size line.
Regards
SHARQ
RE: Relief Valve is causing hammering
I was not clear in use of vented, as to atmosphere.
My point was that the line requiring pressure relief must have some energy storage for the relief valve to work properly. Just substitute the phrase "line requiring pressure relief" for any phrase using "vented" in my previous post to gain my meaning.
PUMPDESIGNER
RE: Relief Valve is causing hammering
Generally the chatter is caused by a series of steps:
- the PSV set pressure is exceed, but the intial slow movement of the valve results in an over pressure in the main pipework (relief valves are also sized with a minimum of 10% overpressure).
- the losses in the inlet pipework and the valves accumulation also allow the pressure in the main pipework to rise further
- the growing flow capacity as the valves opens, the losses in the inlet pipework and the momentum of the opening valve usually result in a fall in pressure at the relief valve inlet and the valve starts to close
- once the valve has closed the flow towards the valve impinges on the closed valve and the pressure rises rapidly - restarting the cycle.
The main factors which make the chatter worse are;
- the wave travel time form the main pipework to the relief valve. The slower the wave speed generally the longer the chatter cycle. The wave speed is a function of pipewall material, pipe diameter and wall thickness, and also to some extent dependant upon fluid properties
- distance from the the main pipework to the relief. The longer the distance the worse the chatter and overshoot
- the diameter of the feed pipework. High velocities associated with a smaller diameter make the surge pressure and hence the chatter far worse
- the losses (bends, tees etc) in the feed pipework. Increased losses tend to affect the chatter amplitude
- the valve design, large inertia componets like the plug/disc tend to cause overshoot and chatter, the effects of the fluid jet impinging on the plug/disc may also lead to asymmetric operation
- vapour pockets can be formed downstream of the valve as it snaps shuts, which then help acceleration of the fluid once the valve re-opens
A far better solution is to use a purpose designed liquid surge relief valve, like the Mokveld (www.mokveld.com) pilot operated surge relief valve. It exhibits a faster reponse than common PSV's and modulate stably around the required relief flow. However even this sort of valve could exhibit some instabiity if the distance from the main pipework is inordinately long.
Former Du Pont de Nemours conducted relief valve chatter testing. The study is documented in PVP-Vol 237-2, Seismic engineering - Volume 2 ASME 1992 (attachment 6). The document title 'RELIEF VALVE CHATTER TESTING'.
Hope this helps.
RobV
RE: Relief Valve is causing hammering
http://www.bellgossett.com/Press/good.htm
RE: Relief Valve is causing hammering
Also, remember that LIQUID relief valves technically DO NOT CHATTER, only relief valves handling compressed vapors and gases can truly CHATTER; and CHATTER is not possible if the valve re-closes.
Now I will confuse you more, a liquid relief valve can be way oversized for its required flow and not have enough stored energy to keep the valve from re-closing; and therefore cycle OPEN/SHUT rapidly which many persons typically refer to as chatter. Again technically this is a mistake, but I have nearly given up trying to correct them ,too many people (including vendors) are using the term incorrectly. True CHATTER for a relief valve occurs when it remains open and the plug is resonating above the disk slaming into the disk which destroys theinternals rapidly; versus rapid opening/closing of the valve which can go on for an extended period of time. The latter is not chatter but we all refer to it that way.
Your "chatter could also be driven by some other source other than the relief valve being too large versus the required flow and available stored energy as aluded to by several previous posts.
Relief Valve Chatter (or rapid OPENING/CLOSING of a relief valve) for COMPRESSED VAPORS AND GASES is normally caused by:
1. A relief valve being several sizes over-sized versus the required relief flow. This is bad because the relief valve wants to relieve at its capacity at full lift(not its required capacity).
2. Relief Valve Piping (inlet and/or outlet) being too small for the max flow (where again pressure drop should be calculated at r/v capacity, not required relief flow)
3. Relief Valve Piping (inlet and/or outlet) being too long rsulting in excessive pressure drop. Note that inlet loss is much more critical than downstream loss.
4. Resonating conditions may occur if the piping system and the relief valve internals (spring constant, spring weight, etc) match. THIS IS THE TRUE DEFINITION FOR RELIEF VALVE CHATTER!
5. Distance from pressure source to relief device is excessively long such that the response time is excessive; and/or the difference in pressure (ie. pressure drop) is excessive. This is where a PILOT VALVE MIGHT BE OF VALUE!
6. I cannot give a general rule of thumb, but remember that the relief valve set pressure also has an effect as to whether a relief valve is susceptable to chatter (function of spring weight).
Regards,
Charlie
The more you learn, the less you are certain of.
RE: Relief Valve is causing hammering
1- Relief valve (Setting, size, etc..)
2- Relief liquid (220 Deg c) is injected to a much colder system line. The distance of the outlet pipe to the main line is 25m.
What we have noticed is the shaking of the outlet pipeline when PSV is opened. Operation suggested to further extending the outlet line to 70m to give a chance to the relieved liquid to cool down and tie it to a line that has a higher temperature than the existing one.
I cannot figure out which is causing the problem and I am working to study the root cause of it. All the remedies written above will help me to investigate on the PSV. I need from you kindly to advice me with analysis of thermal hammering possibilities.
I deeply appreciate your help
Regards
SHARQ
RE: Relief Valve is causing hammering
HALLO EVERY BODY.
IF SOME ONE HAVE IDEA TO TRANSLATE THE WAVE RESORANCE IN WATER NETWORK TO EXACT VOLUME OF PRESSURE VESSEL.
PLEASE ANSEWER.
THANKS FOR ALL.
ENG AHMED
RE: Relief Valve is causing hammering
Are you confused enough?
ER