Relief Valve is causing hammering 2

[SHARQ]

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

 

[Latexman]

An oversized relief valve will "chatter". "Chatter" is the rapid opening and closing of a relief valve. The vessel builds up to the set pressure. The relief pops open. The vessel depressurizes. The relief slams shut. The vessel builds up to the set pressure. And on and on and on . . .
Is that what's happening?


Good luck,
Latexman

 

[SHARQ]

Latexman,

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

 

[PUMPDESIGNER]

Consider how much energy is stored in the vented line, or put another more practical way, if the vented line is short, level, and steel, even one single drop of fluid vented drops the pressure to zero because there is no energy stored in the line.

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

 

[SHARQ]

Hi PUMPDESIGNER

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

 

[PUMPDESIGNER]

Hello SHARQ,
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

 

[RobV]

It seems that you may be encountering some form of pressure surges in the main pipework. Approximately 40% of safety relief valves in liquid lines Susceptible to surges exhibited unstable operation. This is a recognised problem which is often solved with a true liquid surge releif valve.

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 (

http://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

 

[EGT01]

Is the hammering occurring in the relief valve outlet piping? Is the relief valve outlet piping liquid full? At the stated relief conditions, you will likely have vaporization through the relief valve or in the outlet piping. I wonder if you are not experiencing the classic steam/water hammer that is associated with condensate return lines. Maybe you can use some of the techniques for condensate systems for your relief valve.

http://www.bellgossett.com/Press/good.htm

 

[CHD01]

You do not provide sufficient information to determine what is causing your "chatter"!

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.

 

[SHARQ]

There are two reason may are the cause of 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

 

[ASHM200444]

AHMED,
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

 

[GenB]

It can be the valve size/pressure against and overpressure or rapid discharge (way above the valve relief capacity in pressure and capacity) you should analize and re-size the SV based on the BTUH of the liquid discharged,(and total generated by the system heater),its temp and pressure (also pressure generated by the circulating pump), if the pump keeps running after the discharge and at what pressure?.If the pump is rated for greater pressure than your system, it will over-excede the pressure of theSV and there is your problem, the SV, discharge piping and even the equipment will shatter.
Are you confused enough?
ER