5 replies to this topic

[LV Raman]

Hi there,

I have a typical problem in selecting a suitable Pressure and Vacuum relief valve in one of my projects. I have attached a simple sketch showing the actual design conditions.
This is an atmospheric storage tank storing Acetone. The inlet to the tank is from the plant and the outlet is connected to ship loading thro' a set of pumps. The nitrogen is used as blanketing medium.

As per the P&ID there is a breather valve whose outlet is connected to the Flare system. The maximum back pressure is 65 mbar and the over pressure to be set @ 90 mbar. The issue is as per the Breather valve vendor the valve shall operate at 155 mbar (90+65) only.

This is not acceptable to Process engineer for obvious reasons. In other words, it is not possible to countereffect the super imposed back pressure because of the valve design which uses pallets as counter weights.

I would like to get an opinion from the experts for a suitable Breather valve design and, of course, if any of the members had similar problems.

It would be of great help If I can get some catalogues.
Regards
L.V.Raman

Attached Files

•  Storage_Tank_PressControl_Rev1.zip   444.49KB   249 downloads

[JoeWong]

Some questions :

Is the "mbar" in gauge or abs ? Dealing with low pressure setting shall pay extra addition to the unit.



Is the superimposed back pressure vary from 0 to 65 mbar (g or abs ?) ?
What caused this superimposed back pressure ?

What is the design condition (vacuum / overpressure) of storage tank ?

You stated that "...not acceptable to Process engineer...". What are the problems ? The informations may not lead me to drop any comments

Not sure if the set pressure "C" can be reduced ?
Not sure if the superimposed back pressure can be reduced ?

You stated that "...problem in selecting a suitable Pressure and Vacuum relief valve...". There are plenty of PVRV works in the same way. The pressure setting is not extra ordinary. Is it the problem with PVRV or the setting of PVRV ?

[LV Raman]

Some questions :

Is the "mbar" in gauge or abs ? Dealing with low pressure setting shall pay extra addition to the unit.

In gauge

Is the superimposed back pressure vary from 0 to 65 mbar (g or abs ?) ?
What caused this superimposed back pressure ?
The superimposed back pressure is beacuse of the outlet of the Breather valve is coonected to flare header

What is the design condition (vacuum / overpressure) of storage tank ?
The design conditions are 180 mbarg/-10 mbarg.

You stated that "...not acceptable to Process engineer...". What are the problems ? The informations may not lead me to drop any comments
The problem is with the elevated set point because the Breather valve vendor says the valve will releive only at 155 mbarg(90+65) rather than @ 90 barg setpoint

Not sure if the set pressure "C" can be reduced ?
Not sure if the superimposed back pressure can be reduced ?
I am afraid that is not the issue

You stated that "...problem in selecting a suitable Pressure and Vacuum relief valve...". There are plenty of PVRV works in the same way. The pressure setting is not extra ordinary. Is it the problem with PVRV or the setting of PVRV ?

The issue is about selecting a suitable Breather valve; i.e., in the case of a PSV the effect of superimposed back pressure is minimised by the bellows design; this is not possible with conventional breather valves which do not have bellows.

SO THAT WAS MY QUESTION FOR A SUITABLE BREATHER VALVE DESIGN WHICH COULD COUNTER EFFECT THE SUPER IMPOSED BACK PRESSURE.
This will enable us to retain the set point @ 90 mbarg.

[djack77494]

SO THAT WAS MY QUESTION FOR A SUITABLE BREATHER VALVE DESIGN WHICH COULD COUNTER EFFECT THE SUPER IMPOSED BACK PRESSURE.
This will enable us to retain the set point @ 90 mbarg.[/i]

What you call a breather valve is what I'd normally call a conservation vent. I think I appreciate your problem. Normally the vent would discharge to atmosphere and there would be no superimposed back pressure. In your situation, you want to discharge into a flare header which could be anywhere from 0 to 65 mbarg pressure. Thus the differential pressure that the conservation valve would see if it were set at 90 mbarg could vary from 25 mbard {90-65} to 90 mbard {90-0}. (Note that the "d" is emphasized to indicate DIFFERENTIAL pressure. It is VERY important to clearly indicate whether a pressure is gauge, absolute, or differential.) An approach which you may find acceptable could be to set your conservation vent, which is a differential pressure device, for 25 mbarg. Then, even if your flare header pressure were at its maximum value of 65 mbarg, the conservation vent would still begin to discharge at 25 + 65 = 90 mbarg. If the flare header were at a lower pressure, then the conservation vent would begin to open at a lower pressure, but in no case would it be lower than 25 mbarg. If this is not acceptable, then you will need to investigate a more exotic device. Anderson Greenwood makes some pilot operated valves that would probably be what you'd want. Good luck.

[JoeWong]

When i first asked the causes of superimposed back pressure, the intention is to see if you have opportunity to provide large line and connect it closer to flare KOD / tip so that the superimposed back pressure can be reduced.


"The design conditions are 180 mbarg/ -10 mbarg."

"The problem is with the elevated set point because the Breather valve vendor says the valve will releive only at 155 mbarg (90+65) rather than @ 90 barg setpoint"

From pressure perspective, 155 mbarg + overpressure of 10% still lower than 180 mbarg. Should not be a problem. However there is some uncertainties in the PVRV (Breather valve) which possibly lead to the pressure very close to 180 mbarg.

Vendor said PVRV will relieve only at 155 mbarg. This implies that the PVRV is ordinary weighted pallet or spring loaded type. The main problem with these type of PVRV is there is high overpressure in order to achieve full lift PVRV. With the first opening pressure of 155 mbarg and design pressure of 180 mbarg, a very large weighted pallet or spring loaded PVRV may be required.

In order to have a low overpressure (10% overpressure) and optimum size PVRV, a pilot operated PVRV (POPVRV) is recommended.

Concerning the relieving pressure, you can have the POPVRV starts to open at 90 mbarg regardless the superimposed back pressure is 0 or 65 mbarg or from 90 mbarg to 155 mbarg. or 25 mbarg to 90 mbarg. That subject to the design and setting of pilot. Vendor can advise you the details.

Typical vendors are :
i) Anderson Greenwood... http://www.andersongreenwood.com/
ii) Protectoseal... http://www.protectoseal.com/
iii) Groth... http://www.grothcorp.com/



HTH.

[LV Raman]

Gents,
Many thanks for the response.
Regards
L.V.Raman