Pressure Valve Chattering 6

[erdemm]

Hello,

I am still working on a project about "chattering" in pressure relief valves.

My question is:
1) What is the "main" cause of chattering? (pressure loss within the valve?)
2) What are the other important causes?
3) How do those causes make the system to chatter?

Thanks in advance
Erdem Ogut

 

[BobPE]

erdemm:

The primary reason for PRV chattering is incorectly sized PRV's....

BobPE

 

[TBP]

erdemm - Do you mean pressure reducing valves, or safety/relief valves?

 

[erdemm]

Both are acceptable TBP
I would be glad if you could explain the questions for both.

BobPE, thank you for your answer.

 

[psafety]

erdemm,

One of the main reasons I have found for chattering of a safety valve is improper line sizing. Specifically, the inlet line to the safety valve. What's happening is if the system sees an overpressure the safety valve opens (fully, assumes a true safety valve with huddling chamber). The valve will remain open as long as it is at app. 90% full capacity. If the line sizing restricts this flow the valve closes. Pressure builds, it opens fully. The process is cyclic and the rapid opening/closing prematurely wears down the valve nozzle and seat.

Don Coffman

 

[hacksaw]

agree with psafety, inlet line losses are a main contributor.

The valve pops on static pressure increase, the flow losses cause it to reseat before the system pressure (at the source has cleared) and the cycle repeats...

look for reducers, excessive bends and elbows, and valving in the inlet line. also look for long piping runs beteen the relief valve and the vessel or the pipe being protected.

usually once the piping loos during full lift are identified, the reason for chatter is obvious.

good luck,

 

[TBP]

I also encounter a lot of general industry plants that try to operate at or very near to the safety valve settings. It's very common for the unknowing to select a safety valve that's set for their operating pressure, and have it "feather".

 

[BobPE]

you can take into account line and minor losses in system where PRV's are installed, it would not be practicle to minimize line and minor losses only to cut the pressure loss you saved with the PRV valve.....

To take these into account you have to properly size the valve(S). I hinted to a big blunder I see often. A PRV may consist of more than one valve depending on what you are trying to accomplish....

BobPE

 

[jay165]

Why not use a pilot operated PSV with the sensor located on the vessel (or whatever it is you're trying to protect)? That should help with pressure drop on the inlet side. This may stop the chattering, but you may still have a problem if the line is too small, ie, the pipe and valve won't pass enough fluid to bring the pressure down fast enough.

 

[steam4]

If this is referring to a safety valve on a steam system, the culprit may just be a grossly oversized valve.

Or......if this is a steam pressure reducing valve, check out the catalog information for the Watts Model 152A which has an anti-chatter adjusting screw on the side.

 

[TBP]

steam4 - I always thought "steam system" and "grossly oversized" were pretty much synonymous

 

[steam4]

TBP - Hmmmmm......I better add another safety factor...

 

[JAlton]

CAUSES OF CHATTER, CORRECTIVE ACTION from most likely to least likely.
# 1)PRV Oversized for the Installation
- Flow is < 25% of Rated Capacity
Check Required Capacity & Replace PRV if necessary
# 2)Inlet Piping of Excessive Length
- Pressure Drop > Blowdown
Redesign Inlet Piping so that Pressure Drop
is less than 3% of Set Pressure
# 3)Inlet Piping Undersized for PRV
- &quot;Starving&quot; the PRV
Increase Inlet Piping to at least Inlet Size of
PRV Outlet
# 4)Piping Excessive Length
- Built-Up Back Pressure
Redesign Outlet Piping so that Back Pressure does
not Develop
# 5)Outlet Piping Undersized for PRV
- Built-Up Back Pressure
Increase Outlet Piping to at least Outlet Size of
PRV
# 6)Adjusting Ring Setting
- Upper Adjusting Ring Too High
Lower Upper Adjusting Ring to Even With or
Slightly Below Seat Level


J. Alton Cox
President
DeLuca Test Equipment

http://www.delucatest.com

 

[JAlton]

I made a mistake in reformatting for this forum
please see items 3 & 4 below:

CAUSE OF CHATTER, CORRECTIVE ACTION

# 3) Inlet Piping Undersized for PRV
- &quot;Starving&quot; the PRV
Increase Inlet Piping to at least
Inlet Size of PRV
# 4) Outlet Piping Excessive Length
- Built-Up Back Pressure
Redesign Outlet Piping so that Back
Pressure does not Develop

The word &quot;outlet&quot; should have begun Item 4 rather than ending Item 3. Sorry for the mistake.

J. Alton Cox
President
DeLuca Test Equipment

http://www.delucatest.com

 

[BobPE]

JAlton:

Pipe size has nothing to do with PRV size, this is the biggest mistake you can make...And it may be a source of the problem here....

I am not sure what you mean by back up pressure, but chattering PRV's cause downstream pressures to equalize above the PRV downstream pressure setting, this has nothing to do with downstream losses.

A big problem I always see is that PRV's are added onto a system after the system has been in service for a long time.. People try to get too much flow from the PRV by oversizing it. You comments are valid if that is the case and you need large flows, this is usually measured by available pressures in the system, not with flows though...

I am not riding you, this is a great thread and the subject is a very common problem out thre which usually means there is a lack of information and understanding about the process....

BobPE

 

[JAlton]

In addition to My experience over the past 30 years, it is the opinion of the ASME that Pipe size has something to do with PRV size. ASME Sec I, Power Boilers, requires the inlet and outlet piping be at least the the size of the PRV Connection.

This is a quote from a proposed new ASME Sec XI for Transport Tanks, &quot;TR-130 INSTALLATION REQUIREMENTS
(b) All piping, between the pressure relief device and the vapor space of the tank, shall not be less than the size of the pressure relief device inlet to ensure that flow capacity is not compromised. When two or more required pressure relief devices are connected to a single opening, the internal cross sectional area of the opening shall not be less than the combined inlet areas of the pressure relief devices connected to it.

Both Farris Engineering and Dresser Consolidated Troubleshooting Charts refer to Undersized Inlet Piping as a major cause of Chatter. A quote from Crosby, &quot;A valve should never be installed on a fitting having a smaller inside diameter than the inlet connection of the valve.&quot;

According to API RP576, 5.2.c., &quot;Improper or lengthy piping to the PRV inlet…can cause a PRV to chatter. The pressure under the seat may become great enough to open the PRV. However, as soon as the flow is established, the built-up pressure drop in the connecting piping may be so great that the pressure under the seat falls and allows the valve to close. A cycle of opening and closing may develop, become rapid, and subject the PRV Seating Surfaces to severe hammering, which damages the seating surfaces, sometimes beyond repair.&quot;

Regarding Built-Up Back Pressure in the Outlet Piping,
Farris Engineering Manual 192-T states the following:
Problem: Chatter.
Cause: The outlet piping is of excessive length or the diameter is less than the valve flange size.
Corrective Action: Design outlet piping so that pressure on the outlet does not build up causing the valve to shut.
Consolidated Maintenance Manual CON-2 states:
Problem: Chatter.
Cause: Built-up Back Pressure
Corrective Action: Check outlet piping

BobPE you acknowledge Oversizing as a cause for chatter.
If you think about it logically, reducing the inlet piping or making it excessively long both have the same effect on the operation of a PRV as placing a large, high capacity PRV in a low volume application (oversizing). Basically there is insufficient volume to maintain the reaction force in the PRV Huddling Chamber necessary to overcome spring force.

J. Alton Cox
President
DeLuca Test Equipment

http://www.delucatest.com

 

[BobPE]

Jalton:

I know of the ASME code, its one of the main culprits of a lot of the problems I find, no engineer and a cope of ASME. Dont get me wrong, its good they are there, but they are guidelines which do not do a very good h=job for PRV's in my opinion....

The PRV is designed for a flow range, if the system cannot deliver that flow then the valve is oversized, no matter what the diameter and if you design only on daimeter without understanding the system, it is bound to fail...Volume of flow through the valve has nothing to do with its operation, its only the pressure that performs the work to modulate the valve.

It s a common problem and 9 of 10 that I fix were &quot;code&quot; designs....The way I always look at is the codes are guidelines for engineering, not replacements for it and I regularly superceed code when situations warrent....and engineering trumps code...

just my 2 cents

BobPE

 

[JAlton]

I understand that Site Specific Engineering Solutions are preferable to a CODE which seeks consensus. However,I have been Setting & Testing Pressure Relief Valves for ASME Sec. I, Steam and for ASME Sec. VIII Steam, Air/Gas and Liquid Service for 30 years. I have tested on a range of Test Benches from small Test Rigs with no volume to Test Benches with 100 cu ft Test Vessels as well as on Power Boilers upto 2970 psi with Live Steam. My experience with the Performance of PRVs tested on Low Volume and then placed in High Flow Service Applications proves to me that Flow is as important to PRV Performance as Pressure.

There are Code requirements for Capacity Certification which subject sample PRVs to actual flow testing. Prior to such test the PRV must demonstrate Set Pressure and Blowdown as well as Consistent Operation (No Chatter). My experience is that PRVs which fail to demonstrate Proper Performance also fail to deliver the required capacity.
In my case, Experience trumps Engineering.

I have known many of the members of ASME Sub-Committee Safety Valve Requirements for years, many are employees of PRV Manufacturers and all are very knowledgable. They take their Committee Work seriously and have written the requirements to reflect their collective experience.

PRV Manufacturers consistently advise users NOT to attempt setting Blowdown on PRVs with Low Volume Equipment because they know that without sufficient volume the PRV will not perform properly.

J. Alton Cox
President
DeLuca Test Equipment

http://www.delucatest.com

 

[JAlton]

BobPE,

I was re-reading our conversation and I have a question.
When you use the Abbreviation PRV are you referring to Pressure Reducing Valve or Pressure Relief Valve?

All my comments are directed toward Pressure Relief Valves. I have no practical experience in Pressure Reducing applications and I began to wonder if that is why we seem to be in disagreement over some very basic issues.

For years the ASME Code referred to Safety-Relief Valves (SRV), but recently the National Board Inspection Code has attempted to change the term to Pressure Relief Valve (PRV).

I apologize for any misunderstanding.

J. Alton Cox
President
DeLuca Test Equipment

http://www.delucatest.com

 

[BobPE]

JAlton:

sorry for the confusion, I reread your posts too....I was DEFINATELY not talking steam!!!! Nor relief valves.... LOL it is confusing and I wish we engineers would settle on terminology....I was talking pressure reducing service for fluids.....I was wrong not to quantify my response.....my comments only go for fluids for whoever is reading.....

The great thing about these forums is we can figure out what to do and where the confusion is in our industry...I am for ever trying to talk to other engineers in our small world to agree upon terms so as not to confuse people....LOL....and I may have just fallen victim to the confusion....

BobPE