BYPASS, ISOLATION OF PSV WHILE UNIT IN SERVICE 1

[jrwchem]

Hi,

could any one tell me if the relevant codes allow for the following situation:

A psv needs to be serviced while the unit is still operating. Therefore, isolation valves are installed and a manual bypass line is used. An operator stands by on radio and opens the manual valve in the case of an emergency.

If this is allowed, then are there any suggestions on how to size the bypass line and valve?

Many thanks,
jon

 

[joerd]

There is no code I know of that will allow such an operation. Would you want to be that operator standing there waiting for trouble?

Cheers,
Joerd

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Samiran]

If the philosophy is to have a PRV serviced when the plant is on-line then a standby PRV must be provided. You take a risk operating in an isolated mode. However, operating plants frequently have these scenario - for which the best tool is to carry out a risk assessment and present it to the management. Let the risk level decide the decision. This is what we call a probabilistic approach as against a deterministic approach .

 

[jrwchem]

Hi,

I did find an appendix M in the ASME section 8 code that does allow a psv to be isolated as long as somebody is dedicated to watching the pressure and has a means of relieving the excess pressure.

However they are unclear and dont state where the operator has to be during their shift. If they are on location, then what happens if there is a fire?!

I agree, that this is not the best solution. The best solution would be to have an extra psv already attached, and the spare psv can be isolated , or on a selector valve.

Is there any further clarification, experience out there on this issue?

Thanks!

Jon

 

[chemjock]

The bypass lines are very short no more than a few feet. I usually select a block valve and inlet line the same size as the PSV inlet flange and the outlet line the same size as the PSV outlet flange. The bypass doubles as a vent. The operating procedure for this arrangement needs to be developed with a view to the points raised above.

 

[proinwv2]

I can not imagine that this is a desireable situation in that the operator is human and subject to those frailities. While valves are certainly subject to failure, they aren't subject to errors in judgement, not paying attention, or having to relieve themselves, away from the tank.

For safety of personell and equipment, either shutdown or alternative venting are the preferred methods.

Paul Ostand

http://www.ostand.com

 

[mrtangent]

Some valuable comments above.

What is the sizing basis for the relief valve. IE. Is the vessel in the fire zone? What is the valve protecting against - what is documented in the sizing case?

If you decided to proceed you must quantify the risks. I would suggest using risk assessment.

For your info. I have personnaly installed a valve below a relief valve. But we found the risk unacceptable to change the valve while the plant is running but having the valve allowed a much shorter shutdown while the valve was changed and the plant was shutdown. We used a castle key system to ensure isolation of the energy sources.

I've also heard of on-line testing of rv valves, have you considered this ? i think its called trevi testing

Or are you certain that the servicing is required?what the driver?, have you a history of RV's losing their set pressure for this service or have the valves been reliable, could you extent the test interval.

Another option I've seen used is a BD by-pass system for RV while they are being serviced.

 

[jte]

jon-

I'm jumping in here a bit late, but part of the answer to your question depends on the Jurisdiction you're in. Where I work we routinely change out safety valves on the run when their servicing period is shorter than our turnaround cycle. We have an established procedure for doing so. The requirements for us are pretty clearly spelled out by State law:

Stop valves may be installed between a pressure relief device and the operating vessel for the purposes of inspection, repair and/or replacement of the pressure relief device. Where stop valves are installed, the employer shall develop, implement, and maintain a written plan containing all of the following:

(A) Procedures to ensure that stop valves are open and locked or sealed during normal operations and are not to be closed except by a qualified person(s).

NOTE: In the case of multiple relief device installations having spare capacity or spare relief valves, the stop valve can be closed on the inactive relief valves providing sufficient system relief capacity is maintained.

(B) Procedures to minimize the frequency of closing stop valves while the vessel is in service. Stop valves shall remain open and locked or sealed until a replacement pressure relief device is available for installation.

EXCEPTION: Stop valves may be closed for emergencies such as pressure relief device failure.

(C) Procedures to ensure that a replacement pressure relieving device or needed replacement parts are readily available prior to closing the stop valve and removing the pressure relief device. An index for the location of all stop valves subject to this section shall be maintained.

(D) Before the removal of a pressure relief device from operating equipment, facility management shall review and approve a written operations plan for closing the stop valves. The plan shall incorporate, at a minimum, the following elements:

1. An engineering review to ensure that no increase in the capacity, pressure, severity, and/or operating condition of the unit or vessel will affect the safety of employees or equipment while the pressure relief device is out of service.

2. Provisions for written notification and operational procedures to unit controllers, operators and other affected personnel, including the safeguards and estimated time the valve will be out of service.

3. Identification of the functional position(s) or employees having knowledge of the operating equipment and pressure relieving device(s) authorized to approve closing a stop valve; and,

4. Identification of the qualified person(s) or functional position(s) authorized to actually close and then reopen a stop valve.

(E) A written overpressure relief plan for each safety relief device shall be developed prior to closing the stop valve. The plan shall include:

1. The means by which overpressure-relief protection will be provided during the time that the pressure relief device is out of service;

2. The procedures to followed to minimize the time that the stop valve will be closed;

3. The operating conditions, limitations, and other safety measures which are required to be observed to protect employees and equipment; and,

4. In the event that the plan requires a standby person to manually operate a vent to the atmosphere, a replacement safety relief valve shall be available for installation prior to closing the valve. Employees shall be protected from hazards as identified by Article 10 of the General Industry Safety Orders.

(F) The written overpressure-relief plan shall be made available to the Division upon request during the course of the work operation to which it applies.

reference:

http://www.dir.ca.gov/title8/6551.html

jt

 

[DiegoMartinez]

Normally on the critical equipments, the following relieve lines are provided:

1- 2 PSVs; one in operation, the other standby
2- A blowdown line with a blowdown valve(failed open) and a ball valve (locked open) and a restriction orifice
3- The bypass line with a ball valve (locked close) and a globe valve (normally closed)

Please note that you were talking about item 3 in your question. This line is never intended for use in normal operation. The intent of this line is only for plant, equipment purge.

Hope this assists.

 

[Armen75]

I have seen this arrangement quite often; the PSV with block valves on either side (with CSO) and a bypass around them.
Again, based on jurisdiction, this is assuming you are allowed to have CSO-ed valves in your safety relieving lines. Certain areas do not permit adding ANY valves in the relieving path and therefore this arrangement is not permissible, even with the CSO's.

As for the size of the by-pass line (assuming you are allowed to install it) you must consider that you are planning on using it as a replacement to the PSV (temporarily) and it must therefore have at least the same orifice or discharge area as the valve it is replacing.

 

[mechprocess]

Under what conditions will these isolation valves be closed? There existence can be justified only if one of the 2 exceptions to ASME Section VIII Subpart UG-135 (d) applies.

1. when these stop valves are so constructed or positively controlled that the closing of the maximum number of block valves possible at one time will not reduce the pressure relieving capacity provided by the unaffected pressure relief devices below the required relieving capacity: or
2. under conditions set forth in Appendix M a) A vessel, in which pressure can be generated because of service conditions b) pressure originates from an outside source.

I believe item 2a suggest that while servicing the relief valve, pressure can be generated in the vessel. Don’t ask me how that can happen but it seems to fall under the same principle as 2b.

I have used an ASME certified three way valve with two relief devices to design for continuous operation during inspection. Such valve is always open to one relief discharge path.

I don’t know of any jurisdiction that will allow a block valve between a vessel and a relief device.