steam pressure regulator and safety valve

[garfio]

A plant expansion has been designed using 150# steam, and all the setting of the steam safety valves in the expansion is at 150#.

Steam for this expansion will be supplied from the existing steam system, that operates normally at 200 psi but has a design pressure of 250 psi.

I am planning to install a pressure reducing valve on the supply line to the expansion, and a safety valve downstream the valve, with a capacity that matches the flow through the pressure reducing valve 100% open (valve failure case) and at maximum upstream pressure. Is this an adequate arrangement and design criteria?

I also have some specific questions:

1. Should I take into account the relieving pressure of the boiler safety valve (250 +10%) to determine the capacity required for the new safety valve?

2. What kind of control valves are a good choice for this application (flow characteristic, plug style)?


any additional input would be appreciated.

Thank you

 

[MJCronin]

laval..

The Code requirement for a relief valve and for system design comes from ASME B31.1.....not any particular "standard practice".

Most state laws and statutes incorporate ASME B31.1 into thier mandatory requirements for boiler and steam system installations.

Everyone should be familiar with state law (ASME Sections I, VIII and B31.1) as well as the requirements of thier insurance carrier prior to modifying steam and/or boiler systems..

- My suggestions only

-MJC

 

[dcasto]

The question is: Does he calculated the amount of steam the pressure control valve can pass based on 250 psig inlet or 275 psig inlet. I've never seen that in the code books, heck the code doesn't even address what failure modes must be looked at let alone that detail.

I don't normally include the 10% because that is a double failure. If your company policy is to protect against such a failure then you should add that in.

 

[StoneCold]

I would go with the 250# inlet pressure. One more comment though. If you are regulating your steam down to #150 and the relief valves are set at #150 you are going to have a problem.

 

[JimCasey]

Most states have adopted ASME Boiler and Pressure vessel code sections 1 (for the boiler) and Section VIII (for the pressure-retaiing parts of the system) as law.

You will have a class break at the regulator. The Regulator must be Class 300, but the system downstream can be class 150 (ASME B16.34)

Failure of the regulator allowing pressure to the downstream system's MAWP is addressed by having the SRV in place.

The SRV should not be set at less than 110% of the normal operating pressure of the system. Otherwise it can leak and deteriorate, and if it opens it will not reclose because SRVs are allowed by section VIII to require 10% blowdown to reseat.

Size the SRV to handle the flow through the regulator if P1 is 250 and P2 is 110% of the set pressure of the SRV.

 

[Joesteam]

As for what PRV, there are many manufacturers. If you have air available we have much success here in NYC with Leslie Controls valves reducing Con Edison Steam (200 psig max) down to safe usable levels.

http://www.lesliecontrols.com/

It is interesting to note that we use a two-stage PRV setup (two valves in series) with a dedicated safety shut-off valve operated by a pressure switch and do NOT use a SRV. this is allowed under the power piping code...

 

[garfio]

Thanks for all the input. I have a few comments:

Regarding MJCronin' comment, it would be applicable if I am under the jurisdiction of B31.1 (Generally up to the first block valve). This is not the case. My tie-in point if downstream from the first valve, so I believe applies B31.3.

Regarding Joesteam comment, I was about to ask if under B31.3 we have other options besides a PSV. For instance, in natural gas systems I've seen two regulators in a row, one of which is called the monitor regulator. This monitor is installed upstream of the main regulator but takes the pressure signal from downstream the main regulator. The monitor is set at a higher pressure than the other so normally is completely open. If the main regulator fails, the high pressure will be sensed by the monitor and could take over (I think in natural gas, it closes for safety). Anyone has seen an alternative for a PSV?

 

[psafety]

My method would be 257.5 as P1 and 165 as P2. I assume you are correct that the first safety is on the boiler, therefore (250 + 3% OP) = 257.5. Yes, Cv @ 100% open

 

[chicopee]

Garfio, unless you have a reheater and a storage tank in the expanded area , I dont believe that you need a safety valve. Safety and pressure relief valves mentioned in ASME code I and VI are to protect fired pressure vessels and superheaters only.

 

[garfio]

It seems that most of the suggestions are about setting a pressure reducing valve at 150# and a safety valve at 165#, to assure this safety valve will not leak. However, StoneCold comment refered to not being convinient to have the reducing valve set at 150# when the safety valves protecting vessels downstream are already set at 150#. A low pressure drop in the system will cause these valves to leak.

Other problem would be that in the event of failure of the regulating valve, the safety valves on the vessels will all discharge with a releiving pressure of 165# (150# + 10%), and at that point the main safety valve will start to open (set at 165#). In this case, would there be over pressure on the vessels downstream?


If the answer is yes, then the only solution will be to set the reducing valve at a lower pressure (because the set pressure of the valves protecting the vessels cannot be changed.

Do you think there will be overpressure and thus, the reducing valve should be set at a lower pressure?

 

[psafety]

garfio,
As I stated earlier:
My method would be 257.5 as P1 and 165 as P2. I assume you are correct that the first safety is on the boiler, therefore (250 + 3% OP) = 257.5. Yes, Cv @ 100% open

To clarify, the header safety is at 150-psig. The max mass flow to determine is based on 257.5 P1 and 165 P2 (with the pressure reducing valve 100% open Cv. 3% OP on Section I valve, 10% OP on Section VIII.

 

[JimCasey]

Safety setting on the boiler is not relevant unless this system is the only system the boiler feeds. If the safety-relief valves in this part of the system are set at 150, then the regulator should not be set at more than 136. (150-10%). The piping constitutes a code pressure vessel.

Capacity of the SRV should be at least equal to the wide open capacity of the regulator if the P1 is 250 and the P2 is 165(150+10% overpressure)

 

[psafety]

Jim,
What prevents this system from being the only system supplied? Let's say all other systems are valved off. Then I say P1=257.5.