Pressure Loss 1

[C26M13]

Hi,

I need some information about the steps require to calculate the pressure loss to pressure relieve valves for steam (condensate)?

Could you please provide me some examples of the calculation?

Thanks

 

[KevinNZ]

Because the flow rate out the safety valve depends on the pressure at the valve and pressure drop depends on the flow rate, you have iterate between the safety valve sizing calculation and the pressure drop calculation.

 

[aliansari]

Pressure drop across a relief valve is not "calculated" separately. If you refer to the sizing equations given in API RP 520 Part I, the orifice size is calculated using the relieving rate (flow rate through the relief valve), inlet pressure (set pressure) and back pressure (barometric pressure in case of discharge to atmosphere or system back pressure if discharge is connected with flare system). As you see, all the essential ingredients (flow rate, pressure drop, orifice bore) are there, built in.

 

[C26M13]

If the line through a series of vessels (from one point to another point in the system). There is a sudden enlargement and contraction? How to determine the hydraulic calculation from the system?

 

[aliansari]

A picture is better than 1000 words, as the saying is. Is it possible for you to share the circuit, you are talking about?

 

[C26M13]

Thank you,

I want to determine the pressure losses to PRV from point to point 2 is acceptable or not?
The problem is if the valve fully open gas blow by will occur and can cause overpressure in the vessel C.

Please find the attached PDF

 

[LittleInch]

Orlando,

I really don't think that's the way it works. If you get gas/steam blow by through the valve marked V, the usual way is to assume a fully open valve and then work out the mass flowrate through the valve when in gas/steam flow. The pressure d/s of the valve you can take as the set pressure / design pressure of the d/s system.

Then you make sure your relief valve can handle that amount of steam gas.

In reality, sure, there will be some pressure loss at full flow, but there's too many variables there to take any credit for it. Doesn't affect the mass flow.

Your issue I assume is where the design pressure break is and why your PSV is located so far away. Moving the PSV sounds like a better option.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[C26M13]

Thank you all for your kind advise.

I am still working on that issue, struggling with the position of PSV (why so far).
I'm trying to build my own spread sheet by calculating the pressure drop in the system.
As you all mentioned above the variables. If I may say, the steps required are:
1) Flow rate through the valves
2) Pressure drop

Since it is steam (condensate), if I want to calculate the pressure drop from point 1 to 2. how can I take the vessels (sudden enlargement and contraction)in to the calculation?

Thank you

 

[georgeverghese]

Pls note that you should also take into account any vessel internals ( exit demisters, feed devices etc) that may be in the path of this relief stream. You should also consider the risk of any of these devices being partially blocked by dirt, corrosion products, high viscosity corrosion inhibitors etc. in the selection of the location of this PSV.

 

[LittleInch]

Orlando,

Save yourself a lot of time and trouble and just move the PSV. I seriously doubt anyone will sign this calculation off as there are just too many variables and unknowns.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[C26M13]

Thank you georgeverhese and LittleInch for your inputs. I will confirm and find out the way to this issue.

I will get back once I get some good way to discuss with you all.

Thanks,

 

[C26M13]

@littleinch: the reason why the PSV installed so far is more about saving cost. In this case, this installed to protect multiple scenarios and multiple vessels.

 

[MortenA]

Orlando1326

As far as i can see nobody has addressed issue 2) - i will try to do so:

The pressure drop (as you will know) is your upstream pressure (e.g. at set-point or set point plus max. allowable over pressure often 10% or 21%) minus your downstream pressure. Your downstream pressure will depend on your discharge system. The drawing that you included seems to indicate discharge to atmosphere - and then the pressure would be ambient (in metric where im most comfortble 0 barg). If its discharging to a flare or vent system some allowance for pressure drop in the piping must be made.

So dP=P_up - P_down

Using your number and assuming allowable over pressure of 10%

dP=6.6 barg-0bar=6.6 barg

In most sizing cases your pressure drop is not important since your valve is choked and thus only upstream pressure maters (API 520 sec 5.6.3) but you should check for that (sec 5.6.2.4).

The max. capacity can be calculated using the API formula and the size of your PSV must be chosen so its more than your design case. Usually you do the reverse, find your design case, calculate your minimum orifice size and select from API 526 the first that is equal or larger than the required size.
Best regards, Morten