29 replies to this topic

[go-fish]

Hi

I have come across P&IDs which show a PCV on the vapor space of propane, butane and LPG spheres. The outlet of PCVs is connected to the flare system. This is in addition to the 2 x 100 % relief valves on each sphere.

I cannot understand the purpose of these PCVs as pressure exerted by any incoming liquid should just condense the vapor back into liquid. And relieving to flare should be required only in the scenarios when the pressure inside the system exceeds the set pressure (design pressure of sphere) of PRVs.

I am not very experienced in this area so may be missing some aspect. Please advise.

Thanks

[go-fish]

Please note there is no nitrogen blanketing on spheres. So i do not envisage any non-condensables or inert gases.

[ankur2061]

go-fish,

As per API STD 2510 - "Design & Construction of LPG Installations"

PRVs on LPG tanks shall be designed to provide adequate flow capacity during fire exposure. It also says that other causes of tank overpressure, such as overfilling and introduction of material with a higher vapor pressure in a common piping system, shall be considered in determining design fow capacity.

For location of PRVs it mentions that:

The pressure relief valve shall be installed to provide direct connection to the vapor space and to minimize liquid carry-over during vapor relief, especially when the tank is nearly full. This shall be achieved by locating the pressure relief valve connections as close as practical to the top of the vapor space.

Hope this helps.

Regards,
Ankur.

[go-fish]

@Ankur

I meant that the P&IDs have been developed with spheres having both PRVs and PCVs, whereas my understanding says only PRVs should be required as the spheres have high liquid level alarms and there are no non-condensables. The spheres have been rated for full design pressure.

Edited by go-fish, 27 March 2012 - 02:17 PM.

[ankur2061]

go-fish,

My apologies for misunderstanding your query. You are right when saying that PCVs may not be required. Even I have not seen PCVs on LPG / Propane spheres in LPG installations that I have visited.

Probably someone who is familiar with PCVs on LPG/Propane spheres can shed more light on this.

Regards,
Ankur.

[Art Montemayor]

Yes, a pressure control valve can be used on a liquefied petroleum gas (LPG). This is done when the storage tank has a MAWP that can be surpassed if the LPG being introduced into it suddenly has a higher content of lights (ethanes, propanes, and dissolved methane). Should this happen the PCV will vent out the excessive lights that could cause the internal pressure to suddenly rise above the MAWP. This is done to avert the action of a PSV - which is normally installed, but intended only for an emergency but not to control the vessel pressure.

What basically takes place is that the PCV acts as a refrigerating action, evaporating the lights portion of the LPG and thereby cooling and lowering the internal storage pressure. Instead of resorting to a low temperatue vessel, the tank is used with the trade off of consuming a portion of the light hydrocarbons in the flare system. In some instances, one can justify the "wastage" of light hydrocarbons by flaring. However, this is something left over from the era of relatively cheap hydrocarbon fuels.

[go-fish]

Thanks Art

In that case, will the set pressure of the PCV be vapor pressure of on spec LPG corresponding to max operating temperature ? This way when the incoming LPG is offspec i.e. more C2s and C3s, the valve will open to maintain on spec vapor pressure.

[S.AHMAD]

1. Having PCV in addition to PSV is a facility that is nice to have. Meaning, if you already have it, just leave it as it is. Because it has advantages from operational aspects. Normally, the problem is to justify for the installation of PCV in addition to PSV.
2. During commissioning, for example, the sphere is normally contains inert gas (normally N2), It is a lot easier to maintain the pressure using PCV as compare to manual venting.
3. For gas freeing for maintenance & inspection of sphere, it is also a lot easier to purge the remaining LPG via PCV.
4. In an emergency of overpressure case, PSV will be the last resort of protection since PCV will function first and thus PSV will never pop unless PCV is insufficient capacity. Please take note that some case where PSV could not sit properly after popping causing leakage. With PCV & PSV combination this situation could be avoided.
5. With PCV functions properly, on-stream maintenance of PSV can be done if facilities (lock valves) available and local regulations allow for such activities.
6. The PCV set pressure, to take advantage of its existence, shall be between that of operating pressure (vapor pressure) and the PSV set pressure. That is, if the set pressure of PSV is normally 10% higher than VP, then the PCV shall be at 5% higher than the VP.

Edited by S.AHMAD, 27 March 2012 - 07:29 PM.

[go-fish]

@s.ahmad

Since PCV will relief first, what will be a suitable sizing criteria for PSV and PCV?

I understand that max relieving rate for PSV would that for external fire and for PCV would be gas flow equivalent to max incoming liquid flow. Correct?

[Art Montemayor]

Do not confuse the scope of work for each of the two devices in question.

The PCV is a pressure control valve that VENTS any excess gas or vapors in the vessel. This is a normal, operating condition.

The PSV is a Pressue Safety Valve that RELIEVES any excess pressure over the set point, design pressure, or MAWP - whichever is lower. This is a safety device and not an operational device. It is designed to operate only in an emergency case - not during normal operating conditions. For that, you have the PCV venting.

Design each device according to its scope of work: the PSV for the maximum, worst case scenario; the PCV for the normal, expected maximum venting rate.

[Robert Montoya]

Dear go-fish,

the purpose of PCV on sphere is keep the pressure inside the sphere at the bubble point conditions @ maximum temperature ambient. For example if the storage pressure is 245 psig the set point of the PCV should be 245 psig in order to keep the pressure inside the sphere @ bubble point conditions. Now regarding the sizing of the PCV shall be equivalent to inlet flow rate in normal operating.

[S.AHMAD]

1. Mr. Art and Roberdani have answered yours question correctly.
2. In order to minimize C3/C4 loss to flare, it is advisable to set the PCV a little bit higher than the operating pressure. In deciding on the PCV set pressure, take into consideration the variation of temperature due to solar radiation and rundown temperature.
3. We do not want to release LPG unnecessarily. LPG is a priceless commodity....... well! sometimes it is a good idea to take the family out for dinner when the LPG bottle is empty.

Edited by S.AHMAD, 28 March 2012 - 07:09 PM.

[S.AHMAD]

Go-fish

1. We should not produce off-spec LPG in the first place.
2. Routine analysis of LPG rundown should be carried out and corrective action should be taken immediately before the sphere is full of off-spec LPG.
3. Under the situation that LPG sphere vapor contains more C2, the PCV can be used to release the vapor to flare. However, this is not a cost effective method in resolving off-spec LPG. Re-processing or re-blending is normally considered if facilities exist for such operation.

[go-fish]

If PCV has to be set to maintain the bubble point inside the sphere and is meant for max normal operating conditions. Does this mean I have to decide the PCV set pressure strictly by the content of the sphere at a particular time.

I have spare sphere for T&I activities which can store either propane, butane or LPG. I am designing the sphere for propane as it has max vapor pressure amongst the three. Therefore, the PSV is set for 300 psi. However, do i need to vary the PCV set pressure based on the max vapor pressure of what is stored in the sphere i.e. 3 different set pressures?

Edited by go-fish, 29 March 2012 - 02:05 AM.

[S.AHMAD]

1. for sphere design pressure and PSV set pressure need to consider the worst case scenario, which is C3 in this case,
2. PCV can be maintained based on C3 operating pressure regardless of C3/C4 composition.
3. LPG sphere does not require pressure control. Therefore, PCV is redundant but nice to have.
4. During normal operation, LPG sphere pressure does not require any control mechanism. The pressure of the sphere is dictated by the vapor pressure which is depending on composition and temperature.
5. PCV is useful only during abnormal operations e.g start-up, commissioning etc.
6. If you dislike it, you can take it out of service.

Edited by S.AHMAD, 29 March 2012 - 02:28 AM.

[Robert Montoya]

Dear S.AHMAD,

The PCV is never redundant. Remember that fluid enters the sphere from a pump or any system, if the pressure increases within the sphere will be operations problems because the pump (upstream the sphere) is calculated for one specific pressure of the liquid in the sphere, the same applies at time of discharge, if the fluid goes from the sphere to a pumps, high pressure in the pump suction causing possible damage to the pump seal.

In others words the PCV is for operations purposes (control pressure).

[S.AHMAD]

Dear Roberdani
1. I completely understand your comments.
2. In the previous refinery I attached to, we have no PCV on the LPG sphere. In this refinery, any pressure build-up due to excessive C2, the vapor will be vent to flare using a 2" vent line.
3. In the current refinery I attached to, we have PCV that is connected to crude unit overhead drum. This PCV is only being used during "abnormal operations". It is operational useful in the sense that operator does not have to go outside and manually operate the vent. We can easily vent using the PCV from control room.
4. It is for this reason that I said PCV is redundant. Meaning, we can live without it but "nice to have" (Operator will love it)..
5. With regard to pump and sytem design, we should consider the worst case scenario. If we anticipated that 100% C3 will be stored in the sphere, we should design the system as such. The same thing with temperature variations.
6. If you completely understand the relationship between sphere pressure, vapor pressure, composition and temperature, I believe you will be more appreciative of the massage that I am trying to deliver.
7. Appreciate your comment. TQ

Edited by S.AHMAD, 30 March 2012 - 02:11 AM.

[Robert Montoya]

Misunderstanding S.AHMAD;

Dear, your message is very well made ​​and take him into account and thank you very much for the observation "appreciative" I will take it into account in the future. I apologize if my comment was harsh

[kkala]

Reading this interesting thread, I am thinking of another use for the PCV. Its set pressure is assumed to be in the DCS (not locally), so it can be manually readjusted. So PCV could be also used for depressurizing during an emergency (e.g. fire case).

[ankur2061]

kkala:

No, it cannot be used for depressurizing until it is sized for depressurization based on API STD 521 guidelines which requires that the vessel pressure be reduced to 50% of the design pressure from its maximum operating pressure in a time of 15 minutes. You cannot arbitrarily assign another duty to a PCV designed for something else. This is where hardcore design experience comes into picture.

[fallah]

go-fish,

Mentioned PCV is to be installed to maintain the normal operating pressure in the sphere.

Then, set point of such PCV would be well lower than that of the PSV on the same sphere.

Fallah

[ankur2061]

Good luck in trying to depressurize an LPG sphere by manually adjusting the PCV set pressure without knowing to what extent the pressure will reduce and whether it is good enough to protect the vessel.

In fact, control valves are not even preferred for emergency depressurization. It is preferred to use on-off valves with downstream restriction orifice (RO) for controlled depressuring to the levels as prescribed in API STD 521.

[mohds23]

Strongly agree with Mr. Ankur's comments.

PCV shouldn't be used for emergency depressurization because it can lead to uncontrolled depressurization and subsequent low temperature embrittlement, high vibration and noise etc..

[kkala]

I think a PCV (with set point in DCS) could be switched into manual mode, acting more or less as a remote operating valve from the control room. The principal question is whether depressurizing of the sphere in a fire case is feasible or not.
A view can be seen at "Cuidelines for the Protection of Pressurized Systems Exposed to Fire" : http://www.scandpowe...m191-203082.pdf

especially para 4.1 .

"Depressurization systems shall be provided for all process segments containing more than 1000 kg of hydrocarbon. If the content of the system is gas or liquid that will vaporize at rupture (such as LPG) , a system for depressurization shall be provided even for smaller process segments, Ref. /3/".

Refer also to API publication 2510 A (Dec 1996), para 2.11 and 6.6.7.3.:
"Generally, vapor depressurizing systems appear to have limited application in LPG storage". This may be due to "requiring a large-size depressurizing system". On the other hand, this can avoid BLEVE since "after approximately 10-30 minutes of direct flame exposure, the vessel usually ruptures violently" (conditions per para 6.6.3.1.1).

Note: There are also other emergency situations, beside fire.

[ankur2061]

kkala,

The OP's original query was related to the suitability and / or function of a PCV on a LPG sphere. There was no mention about any depressurization by the OP. Using the PCV for depressurization during a fire was raised by you.

Post # 22 & 23 provided a response that control valves are not considered to be suitable for depressurization. Instead of accepting that graciously, you are now trying to divert the topic to whether depressurizing a sphere during a fire is feasible or not.

The question that a LPG sphere requires depresurization or not becomes a separate topic in itself and should be raised as such. Don't you think so?

Regards,
Ankur.