PSV sizing for steam Condensate Tank 2

[Sam Kuna]

Hi,
So I am trying to design a PSV for a steam condensate tank with following information:

Operating temp: 120 C (saturated steam coming from plant into it)
Operating pressure: 30 psi
Design pressure for the vessel: 30 psi
Tank: Horizontal (5 ft by 9 ft )
relief Scenario is blocked discharge. Not fire case.

I calculated wetted area based on 60% of the vessel is full and the flow rate is coming at 425 kg/hr at 120 C discharge from PSV.
That gives me a G type orifice.

Another way I am looking at it is: The line entering the tank is 4" and outlet is also 4". the max steam flow through a 4" line is 715 kg/hr at 15 m/s but I think it will be excessive to say the PSV might be exposed to this high flow. Am I correct??

I am wondering if the calculation I am doing is correct? or I have to be more specific?

Any guidance will be appreciated.

 

[Latexman]

It is good practice for steam condensate tanks to also be rated for full vacuum. During shutdown the steam condensate tank can cool down and depending on valving the tank may go under vacuum.

If rated for full vacuum, the tank's MAWP may be calculated as > 30 psig. If so, it is good practice to claim the higher number. It will not cost any extra.

I don't understand what blocked discharge flow rate has to do with wetted area.

Good Luck,
Latexman

 

[Sam Kuna]

The only reason I did the wetted area calculation is to check the flow to make sure its not very high then max flow through the line.
@Latexman: What do you mean by ... [highlight yello]If rated for full vacuum, the tank's MAWP may be calculated as > 30 psig[/highlight]. If so, it is good practice to claim the higher number. It will not cost any extra.

 

[Latexman]

If the full vacuum calculation determines the thickness of the vessel/heads, i.e. worst case, then use that thickness and "back calculate" the MAWP.

Good Luck,
Latexman

 

[The Obturator]

What is the set pressure for the PRV?

Typically this would be same as MAWP but you are advising same pressure for operating and design.

Also advise how you have determined the flow rate.

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[Sam Kuna]

@ the Obturator:

This is the maximum flow a 4" line can handle so basic D-W equation.
The MAWP is 30 psi.
This is an existing vessel and we repurposed it to lower pressure of 30 psi.
the original vessel was designed to 100 psi.

 

[The Obturator]

Set Pressure ? (should also be 10 % higher than operating).

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[Sam Kuna]

The set pressure for PSV is 30 psi.

 

[Latexman]

120 C sat'd steam is 14 psig. Maybe, that's why it works okay?

Good Luck,
Latexman

 

[Sam Kuna]

Thanks.. Does anyone has a calculation sheet or excel sheet to estimate the flow through PSV?? and design of PSV?

 

[TiCl4]

I don't mean to derail the thread, but what is the worst-case relief for a condensate recovery system? Would it be a steam trap fail-open scenario that dumps non-condensed/full pressure steam to the line and tank?

 

[Latexman]

Some/most vendors will supply software that will size and give capacities.

Good Luck,
Latexman

 

[The Obturator]

I can easily do a few calculations using API-520 but we need to get the conditions set

I've done one calculation using 715 Kg/hr as the max flow (steam) due to blocked outlet and with set pressure of 30 PSIG (with 10 % overpressure). In this case the saturated temp jumps to 137'C. The result is a "G" Orifice.

However, the operating pressure is stated as 30 PSIG. There is typically a margin of 10 % between op and set pressure.

I can't comment if the required capacity is being determined right.

Attached is one manufacturers tabulation of saturated steam through various sized orifices. The flow stated is in Kg/hr.

At the moment, Your going to be looking at a 1.1/2" 150LB flanged inlet, carbon steel body PRV. Threaded connection may also be available.

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[The Obturator]

* Correction. I should have stated "H" Orifice.

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[MJCronin]

Latexman, as always, gives the best advice .....

Design for FV conditions and have a relief device vendor help you ....

I am guessing that this is the first time you have ever done this ...... Is that right ?

MJCronin
Sr. Process Engineer

 

[KevinNZ]

Not sure I understand your case but here goes,

The design flow for the SV is the maximum possible inflow. What is the maximum pressure upstream? Are there upstream SVs and what is the set pressure? If the upstream SVs are set at the same or lower than the design pressure of your vessel the no SV required on the tank.

If the upstream pressure can be higher then you need look at

1, How much steam can the upstream source produce at your tank design pressure?
2, can you rely on the pressure drop between the source and tank to limit flow?

note 30psi (2 bar) steam is not 120C