steam line - safety valves 3

[Boiler1947]

Where in the ASME code can I find : when there are 2 or more high pressure boilers connected to a common header and the boilers are of different design pressures that a steam header safety valve(s) must be installed

 

[TBP]

Do the boilers have the same operating pressure?

 

[Boiler1947]

All the boilers will operate at 125 psi ... but two boilers are designed for 250 psi with appropriate safety valves ..one boiler is designed for 150 psi with it's valves set at 150 psi

 

[TBP]

I'd look at changing the safety valve settings on the 250 boilers to 150. I'm assuming that they're firing to maintain a header pressure of 125, which means at high fire, the drum pressures will be 140 PSIG or so.

 

[Boiler1947]

We are looking at doing that ..but we're talking 6 safety valves.. so I was looking at the possibility of putting 2 valves on the header. I knew I could "safety" the header to protect the downstream equipment ..but I wasn't sure how the code handled the different pressure boilers.

 

[DLANDISSR]

Boiler1947:
You can put a relief device on the ajoining piping provided that:
1. That the relief device(s) are of sufficient relieving capacity that they can relieve the total capacity of all of the boilers and that they are set to relieve at the pressure rating of the lowest rated boiler and that when relieving the pressure in the boiler is not more that 10% above the boiler rating.
2. The piping is of sufficient size, i.e. not less than the inlet of the relief device. In this case the piping leading up to the header could be the size of the relief valve on each boiler with the header size being the same size or large of the relief valve inlet. See UG-133 an 134.
3. If you are leaving the relief devices on each of the boilers in tact the individual boilers are protected and there can be intervening valves between the boilers and the relief devices on the piping.
4. Calculations should be done to determine the effect of the piping on the relief capacity of the valves due to frictional pressure drop in the piping which will result in a lower inlet pressure at the valve. This is true of the relief valve outlet piping also.

 

[TBP]

I'd bet that by the time you selected, bought & installed the safety valves on the piping (a shutdown job?), re-setting the 250s to 150s would be the cheapest option.

 

[davefitz]

If the 250 psig boiler was designed to also provide 100% MCR flow at 150 psig, then it is probably a better idea to just reset the 250 psig relief valves down to 150 psig and document the rerating.

It is possible that the 250 psig units were not designed to produce 100% MCR steam flow at 150 psig- the drum steam water separators might have excessive carryover at the lower pressure. In that case, the 250 psig unit would need a presure reuceing valve at the outlet and then additional 150 psig relief valves would be provided at the outlet of the pressure reducing valve, as spelled out in B31.1 or B31.3.

 

[JAlton]

I agree with TPD regarding the cost of resetting the Boiler Safety Valves versus installing new Safety-Relief Valves downstream of a reducing station. However, I also concur with davefitz. It may be necessary to use the reducing station and new safety-relief valves. The Boiler SVs are ASME Sec. I, "V" Code Stamped, which means they are rated at 3% overpressure for capacity. Downstream of a Pressure Reducing Station, the Safety-Relief Valves would be an AMSE Sec. VIII application rated at 10% overpressure per BLANDISSR's comment. J. Alton Cox
President
DeLuca Test Equipment

http://www.delucatest,com