Steam Safety Valves

[fapsa]

I'm working on a new proyect, I want to know how to calculate the proper size for a safety valve for a steam line that has a flow of 8500 lb/hr, we will install a pressure regulator (from 200 psi to 43.5 psi) after the regulator will be a control valve (setpoint 21.75 psi) in this part of the line I need to install the safety valve to be set at 36.25psi. The pipe line size will be 6", the manufater of the equipment recomends a 4"x6", my question is why this size, why not a 3'x4"?

 

[TD2K]

You need to determine the required flow rate through your PSV for the various possibilities: closed downstream block valve, valve failing open, etc.

The 8500 lb/hr has really no bearing on the size of the safety valve you need, it's the upset condition that will otherwise overpressure downstream piping or equipment that sets the required flow through the relief valve.

Once you have established the required flow rate, then you can size the relief valve for which orifice you need. The manufacturer's catalogue will then, for that orifice and the flange ratings you want (likely class 150 inlet by 150 inlet in this case), will tell you what size of relief valve you need that will accomodate the required orifice.

 

[TD2K]

One more addition. Who is the manufacturer who is recommending the PSV size? Unless they have knowledge of the piping system and the operating conditions, they should not be setting the PSV size. That's up to the process engineer or whoever can take a look at the complete system to first identify all the possible failure modes resulting in an overpressure situation, identify the required relieving rates for each case to pick the sizing case.

If you do a keyword search on deareators, there is a discussion where the deareator manufacturer may provide a relatively small connection on their vessel for a PSV, way too small in some cases for the potential relieving requirement. This is because they don't know the steam supply system and therefore the possible flow rate of steam that 'can' flow to this vessel in an upset.

 

[athomas236]

hello,

you need to look at the requirements of the design code you are using and the requirements of the local statutory regulator. that said the flow rate should be that with the suppy presuure at its max value and all upstream vlves fully open
regards
athomas236

 

[sugarspice]

Find out the flow coefficient (Cv) of the pressure regulator. Next calculate the maximum expected flow lb/hr using a Cv equation you can find in pressure regulator valve catalog.

Next, determine what type of relief valve you want. There are basically two types; spring design or pilot design. The pilot design is better if you want relief to occur closer to the set pressure. Find a catalog for the type of relief valve you want, go to a vendor website or call a vendor. From here you can figure the required orifice size for the relief valve. This in turn will tell you the flange inlet/outlet and pressure rating.

 

[saxon]

fapsa, When it comes to sizing a PSV, the worst possible case scenario needs to be taken into account. In your case, from what I've been able to make out, would be, boiler at full load, PRV failure in the full open position, CV failure in the full open position, all downstrem users blocked off. The PSV orifice design will need to be able to relive the entire mass load at the Max. allowable working pressure (set press.). this calc. will also need to take into account any allowable accumulation that may occur.

Hope this helps.
saxon