Safety valves on steam piping to B31.3

[athomas236]

I am involved in a project where some new piping and valves are being added to an existing system all of which is designed to ASME B31.3.

The new piping takes HP steam reduces its pressure and temperature thro a reducing valve and desuperheater and then connects to a LP steam pipe just downstream of an existing isolating valve.

The contractor has recognised that he needs to provide safety valves on the downstream side of the reducing valve to protect the LP system. The point at issue is the capacity of these new relief valves.

The contractor wants to lock open the existing isolating valve so that there will be a flow path to some existing safety valves installed under an earlier phase. In this way the safety valves that the contractor provides will be about half the code requirement.

Any comments on this would be helpful.


Best regards

athomas236

 

[TBP]

If the piping was designed properly, then the safety valve in question was selected, and the point of installation specified. How were the existing safety valves selected? Their capacity is likely already spoken for. In any event, I don't see an inspection authority going for "locking open" an isolation valve.

If nothing else, "locking open" an isolation valve is, to me (coming from a maintenance background), a terrible suggestion, that will cause the maintenance crew problems for years to come. It's always hard to say for sure, without actually seeing the proposed installation, but this suggestion strikes me as having "trouble" written all over it.

 

[athomas236]

TBO,

Thank you for your advice.

My own opinion is that relying on locking valves open is not inherently safe. I will follow up the maintenance problem with the client.

Best regards,

athomas236

 

[tickle]

If you did a HAZOP on the system it will quickly fall out whether the valves can be closed, What the capacity of the existing valves is, and what procedures are in place for maintenance in closing the valves and isolating the different sections of the pipe system

It is quite feasible that the valves are designed against the full steam generator flow. But has been stated earlier, if the relief valves are isolated they are not going to do their job.

 

[athomas236]

tickle

I had forgotten that there is a HAZOP in progress at the moment.

Best Regards,

athomas236

 

[sailoday28]

For protection of down steam piping, consider failed open upstream valves----the mainsteam reducing and/or the desuperheating water source. Either flow with a known maximum downstream pressure will determine the size of a safety/relief valve on the lower pressure piping

 

[athomas236]

sailoday28

Thank you.

When I started this thread I was just trying to answer a simple question about the installation of isolating valves at the inlet to safety valves. B31.1 does not allow this whereas B31.3 does which was a surprise.

The project is an extension to an existing desalination plant. The extension covers 2 more desal units, 2 steam turbines with bypasses, 5 HRSGs, 5 GTs and 2 dump condensers.

The steam for the existing desal units was raised by fired boilers and then let down to about 3bar before being admitted to the desal units. The existing safety valves were installed downstream of the PRVs to protect the downstream piping and desal plant as you state.

For the extension the contractor is proposing additional safety valves downstream of the ST exhausts which are about 50m from the existing SVs. It is the isolating valves at the inlets to the existing safety valves that the contractor wants to lock open. In this way the contractor wants to count the existing SVs as part of the pressure relieving capacity.

The contractor has proposed that the total pressure relieving capacity should be equal to the max ST exhaust flow but has forgotten that with the ST's on bypass the desuperheating water to drop the temperature from 569C to 180C will result in a flow higher than what he is proposing.

In addition wants the valves that control the steam system pressure by dumping steam to the dump condensers to be considered as power actuated safety valves so they can be considered as part of the pressure relieving capacity. These pressure control valves are just standard control valves and have inlet isolating valves that will not be locked open.

Finally, when the desal units are not shut down and therefore their safety valve capacity not available because their inlet iso valves will be closed, the contractor wants the DCS to issue an alarm so an authorised person can monitor the plant and take corrective action in the event of an overpressure.

We continue with our efforts to persuade this major international contractor of the error of his ways

athomas236