4 replies to this topic

[aju_1807]

HI,

WE HAVE STORAGE TANK WHICH IS BLANKETED WITH FUEL GAS. FOLLOWING ARE THE DETAILS FOR THE SAME:

TANK FLUID : HYDROCARBON OIL
BLANKETING GAS : FUEL GAS
MOL WT : 18.97
DESIGN PRESSURE OF TANK : 130 / -5 mbarg
INLET PCV SET PRESSURE : 30 mbarg
OUTLET PCV SET PRESSURE : 80 mbarg
PVRV SET PRESSURE : 100 / -4 mbarg (WITH 10% OVERPRESSURE)
EMERGENCY VENT SET PRESSURE : 119 mbarg (WITH 10% OVERPRESSURE)

THE PVRV OUTLET IS ROUTED TO ATMOSPHERE. I HAVE FOLLOWING QUESTIONS:

1. WHETHER SET PRESSURE OF PCV'S, PVRV AND EMERGENCY VENT ARE OKAY WITH RESPECT TO DESIGN PRESSURE OF THE TANK?

2. THE PVRV OUTLET IS PROVIDED WITH GOOSE NECK. HOWEVER, I PERSONALLY THINK IT SHOULD BE ROUTED TO ATMOS DIRECTLY (WITHOUT GOOSE NECK) TO AVOID THE POTENTIAL IMPENGIMENT TO TANK/OPERATING PLATFORM. ANY VIEW OR GUIDELINE WHICH INDICATE THE CORRECT PRACTICE FOR PVRV OUTLET ROUTING?

3. WE HAVE GAS BLOWBY SCENARIO WHICH WE ARE PLANNING TO HANDLE THROUGH EMERGENCY VENT. TO AVOID TEH CONTINOUS LONG TERM RELIEF, WE ARE PROPROSING TO USE SELF CLOSING TYPE EMERGENCY VENT. ANY FEEDBACK/EXPERINECE OF SELF CLOSING TYPE EV IN PAST?

4. AS THE FLOW RATE IS VERY HIGH (DUE TO NEW API2000), WE ARE INDICATING TO USE STAGGERED SET PRESSURE FOR PVRV(IN CASE VENDOR PROPOSE MULTIPLE PVERV). IS IT REQUIRED TO STAGGERED VACUMM SIDE SET PRESSURE ALSO? PRESENTLY WE HAVE INDICATED TO STAGGER PRESSURE SIDE SET PRESSURE TO 105 mbarg.

5. IS IT ALWAYS RECOMMENDED TO HAVE REMOTE SENSING TYPE PCV'S?

APPRECIATE YOUR PROMPT RESPONSE.

REGARDS,

AJ

[fallah]

HI,

WE HAVE STORAGE TANK WHICH IS BLANKETED WITH FUEL GAS. FOLLOWING ARE THE DETAILS FOR THE SAME:

TANK FLUID : HYDROCARBON OIL
BLANKETING GAS : FUEL GAS
MOL WT : 18.97
DESIGN PRESSURE OF TANK : 130 / -5 mbarg
INLET PCV SET PRESSURE : 30 mbarg
OUTLET PCV SET PRESSURE : 80 mbarg
PVRV SET PRESSURE : 100 / -4 mbarg (WITH 10% OVERPRESSURE)
EMERGENCY VENT SET PRESSURE : 119 mbarg (WITH 10% OVERPRESSURE)

THE PVRV OUTLET IS ROUTED TO ATMOSPHERE. I HAVE FOLLOWING QUESTIONS:

1. WHETHER SET PRESSURE OF PCV'S, PVRV AND EMERGENCY VENT ARE OKAY WITH RESPECT TO DESIGN PRESSURE OF THE TANK?

2. THE PVRV OUTLET IS PROVIDED WITH GOOSE NECK. HOWEVER, I PERSONALLY THINK IT SHOULD BE ROUTED TO ATMOS DIRECTLY (WITHOUT GOOSE NECK) TO AVOID THE POTENTIAL IMPENGIMENT TO TANK/OPERATING PLATFORM. ANY VIEW OR GUIDELINE WHICH INDICATE THE CORRECT PRACTICE FOR PVRV OUTLET ROUTING?

3. WE HAVE GAS BLOWBY SCENARIO WHICH WE ARE PLANNING TO HANDLE THROUGH EMERGENCY VENT. TO AVOID TEH CONTINOUS LONG TERM RELIEF, WE ARE PROPROSING TO USE SELF CLOSING TYPE EMERGENCY VENT. ANY FEEDBACK/EXPERINECE OF SELF CLOSING TYPE EV IN PAST?

4. AS THE FLOW RATE IS VERY HIGH (DUE TO NEW API2000), WE ARE INDICATING TO USE STAGGERED SET PRESSURE FOR PVRV(IN CASE VENDOR PROPOSE MULTIPLE PVERV). IS IT REQUIRED TO STAGGERED VACUMM SIDE SET PRESSURE ALSO? PRESENTLY WE HAVE INDICATED TO STAGGER PRESSURE SIDE SET PRESSURE TO 105 mbarg.

5. IS IT ALWAYS RECOMMENDED TO HAVE REMOTE SENSING TYPE PCV'S?

APPRECIATE YOUR PROMPT RESPONSE.

REGARDS,

AJ

1-PCV is pressure reducing one?If so,there would be no inlet set pressure and just a set point as per PCV downstream pressure.Inlet pressure of PCV would usually be in the order of 6-7 barg and 80 mbarg would probably be PCV set point and tank operating pressure as well.
PVRV set pressure could be adjusted such that with considering overpressure being 130/-5 mbarg.Thus,seems you could increase its set pressure up to 118 /-4.5 mbarg (of course with 10% overpressure).
EV set pressure seems would be OK.
2-In the case of using goose neck,build-up back pressure should be considered in PVRV sizing.
3-Self closing would be OK,regarding safety issues and preventing loss of inventory.
4-Considering staggered arrangement in vacuum side depends on how much is the rate of flow out from the tank.
5-In my opinion not always.It depends on criticality of the relevant process.

Hope above helps you out.

Edited by fallah, 05 June 2010 - 05:25 AM.

[proinwv]

1. Isn't your PCV the blanketing valve? (PRV?) If so inlet would be larger than outlet. Please clarify.

2. You logic and fallah's are ok

3. I like self closing especially where it is reasonable to anticipate opening due to reasons other than fire. Otherwise the vent could be open but not realized.

4. Staggering of the setpoints for multiple vents is acceptable and as well for pressure or vacuum.

5. I insist that blanketing sensing be directly into the ullage space being controlled. Otherwise, with low setpoints piping losses can influence the valve response and lead to cycling, and loss of control.

Further, I see that you are considering the overpressure characteristics of the vents. That is good. Be sure to confirm your numbers with the vendor and also consider the other side, that is the reseat pressure. Avoid interaction between vents and valves.

[djack77494]

5. I insist that blanketing sensing be directly into the ullage space being controlled. Otherwise, with low setpoints piping losses can influence the valve response and lead to cycling, and loss of control.

Paul makes an excellent point here. What is easy to overlook is that the presence of an apparently innocuous little drop of water can have a significant impact. In small diameter tubing, for example, that droplet could collect in a low spot. If it seals the tube and (let's just say) extends over a length of an inch or a few centimeters, then the blanketing valve might "think" that it is operating an inch or a few centimeters higher or lower in pressure than is actually the case. If you're talking about a vessel operating at many psig or several barg, then no big deal. the "error" is "lost in the noise". But if you're talking about an "atmospheric" tank, then the size of the error may approach or even exceed the valve's setpoint.

Edited by djack77494, 10 June 2010 - 02:23 PM.

[proinwv]

Let me expand upon djack's good comment.

There has been more than one tank, vacuum collapsed, because the blanketing valve was below the level in the tank and a few inches of fluid collected in the sensing line produced a false signal, preventing the valve from opening in response to a withdrawl in product (or temperature drop). Add to this a defective vacuum vent and you have a disaster.

If you have never seen the result of this I recommend that you do, by photograph please, because you will then remember it. (No it was not a tank that I was involved with, fortunately ) I found this image on a Google search vacuum collapse