7 replies to this topic

Hi all,

Could any one of you help me out to solve the following problem.

Actually, there are two LPG spheres operating at 15.3 bar g and 60 deg c and having a vapour balance line in between them. Both the sphere are protected by a PSV set at 19.3 bar g and designed for the Fire case. In addition to that, I can see one Pressure control valve relieving to flare. Now I want to size this PCV.

Initially I thought, if there is no flashing (as it is) at upstream, i should size this PCV for the maximum incoming liquid flow rate considering LPG outlet isolation valve closed. Set point at about 17.4 barg which is 10% lower than PSV set point and pressure drop through the PCV shall be 7.4 barg considering a maximum flare back pressure of 10 bar g.

Then I feel or rather have a doubt whether this sizing is ok or not. Pls help me with your suggestions and advice.

rgds
Durdanto

[fallah]

Initially I thought, if there is no flashing (as it is) at upstream, i should size this PCV for the maximum incoming liquid flow rate considering LPG outlet isolation valve closed. Set point at about 17.4 barg which is 10% lower than PSV set point and pressure drop through the PCV shall be 7.4 barg consideing a maximum flare back pressure of 10 bar g.

I think PCV set pressure should be 15.3 barg (operating pressure) and pressure drop through the PCV shall be 5.3 bar consideing a maximum flare back pressure of 10 bar g.

[Rouzbeh553]

Durdanto:

Same as Fallah, I guss PCV SP should 15.3 instead, since it is the only pressure control element of this system. If it is 17.4 then what else can gurantee the mentioned operating pressure (15.3)?

Beside, Althogh these tanks do have thermal insulation layers against heat tranfer and solar radiation, it is necessary to consider some thermal expansion effects for the worst conditions.

please send more details (if any), for example if there is any vapor recovery system or other pressure controllers,...

[Art Montemayor]

Durdanto:

What do you mean by “I can see one Pressure control valve relieving to flare”? Are you actually looking at the PCV or are you trying to say that you are contemplating installing a Pressure Control Valve (on the vapor balance line between the two spheres)?

If a PCV is already installed on the system, I would challenge the need for it. The two spheres are already protected and there should be no need to flare any gas from the spheres during normal operation. Or is it that you expect non-condensables to accumulate in the spheres? Just exactly what is the purpose or the scope of work of the PCV? Once you define the purpose or the expectation(s) for the PCV, then you size it. It is not normal practice to install a PCV to flare on LPG storage tanks. If one exists, then it must be for a specific purpose or reason. If one does not exist and you are proposing one, then YOU must know the reason for installing one.

I see no reason for having a PCV designed for the maximum incoming liquid flow rate. You should be protected by liquid level alarms and switches on the spheres.

Contrary to what Rouzbeh states, there is no insulation on these tanks – if they are, indeed, storing LPG and the operating pressure is 15.3 barg. That is normal, saturated liquid LPG at ambient temperatures. It is NOT “necessary to consider some thermal expansion effects for the worst conditions” by using a PCV. The PCV does nothing for that. Normal LPG storage tank fill % takes care of any liquid expansion inside the tank. The normal maximum fill % is 85-90%.

[Rouzbeh553]

Dear Art,

Thanks for your prompt reaction and I appreciate you for all the things which I've learned these years from you.

I exactly 100% agree with you regarding FULL PRESSURE DESIGN SPHERES.

What I told was in general (we have a pressure controller means we need to control operating pressure),

actually, what I realize from the question was about a PCV on a Sphere and according to what I've seen up to now, it's only rational when we have not full pressure design Sphere. (e.g. with vapor recovery), that's why I asked Durdanto about other details.

Please kindly correct me if I'm wrong.
Thanks

[Art Montemayor]

Rouzbeh:

Thank you for replying in detail. Our Professional Forums are meant and dedicated to the sharing of important or constructive information among engineers – especially when it relates to Public Safety and safety in general. As engineers, we are licensed and/or expected to contribute to human development in a safe and progressive way in order to facilitate and improve the living conditions and economic level of our society – regardless of nationality, ethnicity, politics, or religion. Our fellow humans expect us to resolve their day-to-day problems safely and economically and we should do all that we can to those ends.

My original intent in my response was to alert the O.P. (Original Poster) that the fluid in question was LPG (Liquefied Petroleum Gas), otherwise identified as a saturated mixture of approximately 50% Propane and Butane and that this mixture is universally used as a residential and commercial fuel and is contained normally in pressure vessels designed to withstand 250 psig (17.2 barg) and usually fabricated for a MAWP of 350 psig (24.1 barg). Since the O.P. said that the spheres were “operating at 15.3 bar g and 60 deg c and having a vapour balance line in between them. Both the sphere are protected by a PSV set at 19.3 bar g and designed for the Fire case”, then the information given seems to make sense and fits a typical LPG design - with 60 oC taken as the maximum ambient temperature.

Another, safety, feature of handling and storing LPG is that a vapor space is always allotted to its storage tanks or vessels since it is a pressurized, saturated liquid at ambient temperatures and the stated pressure of 15 barg. Any increase in ambient temperature will immediately increase the fluid’s vapor pressure and the tank must be able to withstand such a variation in pressure safely. That is why the MAWP is usually approximately 24 barg and a vapor space is specified as being approximately 15% of the total. This avoids any ability of the liquid to exert hydraulic pressure due to its expansion related to temperature increases. When and if a fire case appears, the vessel fluid vented is saturated vapor at the top and at its relieving temperature.

Because LPG liquid conveniently fits ambient conditions, there is no reason, normally, to allow inert or non-condensable gases to exist in the storage vessel’s vapor space. Therefore, the pressure existing in the vapor space will normally be the true vapor pressure of the LPG mixture – which is normally below the 24 barg MAWP. HOWEVER, there may be exceptions to this. I have, in the past, applied addition of pure nitrogen into the LPG vapor space in order to increase the NPSHa to the vessel’s LPG pump-out pumps. This was done because the storage vessels, unfortunately, were installed too low in height over the LPG pumps and there was no other way to increase the NPSHa. In cases such as these, then there may be a need for a PCV (pressure control valve) to be installed on the storage vessel’s vapor space to allow it to release any excess inert vapors that may accumulate in the vapor space and cause the total vapor space pressure to exceed the design pressure. Normally, these inert gas purges would be routed to the existing flare system. This is the safe way to compensate for any pressure increases due to inert or non-condensables in the system. Nevertheless, bear in mind that normal LPG does not have any non-condensables or inert gases mixed with it. This inherent characteristic is one that makes it a very safe and conveniently used as a residential fuel that can be safely used by housewives and commercial business without endangering the public. The non-condensable or inert gases have to be either intentionally or accidentally added.

I hope this helps to explain my comments and also reinforces what you are also concerned about. I consider this an important topic and that is why I have gone into much detail. I hope I have not exceeded your patience, and that I have succeeded in explaining what I wanted to expound to the O.P.
Regards,

[sachindhopade]

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[ankur2061]

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sachindhopade,

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