8 replies to this topic

[Shahnawaz Makroo]

I have seen process datasheet for an LPG barrel pump having NPSH avilable mentioned as zero.Can some make me understand why barrel pumps have zero NPSH a and is it possilbe?



Regards
SAM.

[Art Montemayor]

Sam:

All the LNG pumps I have worked with and employed have been submerged.

Practically all LNG being handled today is in the saturated state and can't tolerate any heat or friction pickup without vaporizing. This causes a grave problem on a centrifugal pump requiring a positive NPSHa. That is why these pumps are almost always submerged. You don't describe the pump installation (I don't even know what you mean by "barrel" - other than that is a nickname given to the type of construction), but I believe the "zero" NPSHa label is meant to denote that the pump is to be totally submerged in the saturated liquid.

Gives us all the details (like a copy of the datasheet) and we can comment further.

[Qalander (Chem)]

Dear Sam

Hellot,
Although as usual 'ART' has guided you correct as per his Vast/Versatile expertise back-up that one always admires.Your query will be better responded on seeing the data sheet definitely.
However it's My very little practical observation that

'Submersible/multi-stage centrifugal pumps' are really useful and efficient even with LPG.

These do require a minimal NPSH although not 'zero'exactly.

However I witnessed in LPG service almost complet emptying out the vessels safely possible.
Similar should be the advantages/reasons for 'what has been described by you' I believe.
Hope this helps
Qalander

[beatles4321]

Sam,

Perhaps barell pumps are cryogenic pumps (pot-mounted pumps - where the impeller and motor are encapsulated inside the pot and is exposed with the LNG) in LNG plant. I believe NPSHa is almost not applicable to this type of pump due to the pot design, the impellers are at the bottom most of the pot and the inlet is way above (side inlet). The pot design gives positive NPSHa. Check EBARA cryogenic pumps - this might give you the whole picture.

beatles4321

[shan]

Hi Sam,

As stated in the definition, NPSHa is net positive suction head available, which has nothing to do with the pump design, while NPSHr (net positive suction head required) is dependent on the pump design.

Usually, LNG pump is installed inside the tank rather than below/outside the tank. Therefore, NPSHa is 0 when the tank approaches empty, while it is elevation difference between tank’s LLL and pump center line for the below/outside tank pump installation.

Regards

Shan

[Shahnawaz Makroo]

Thank you all of you for your good and valid suggetions regarding barrel pumps NPSH a.I got the clue from type of installation which is important.

Actually it is an vertical barrell pump (having vertical shaft inside the casing) and whole arrangement is dumped inside the ground with suction and discharge flanges visible above the ground.This pump is used to transfer LPG from bullets and is installed outside the bullet not submerged inside the bullet.

The NPSH a at the suction flange may be set as zero at suction flange as NPSH required by machine is fulfilled the height of barrel and extent to which pump is inside the ground.




Regads
SAM

[Qalander (Chem)]

Thank you all of you for your good and valid suggetions regarding barrel pumps NPSH a.I got the clue from type of installation which is important.

Actually it is an vertical barrell pump (having vertical shaft inside the casing) and whole arrangement is dumped inside the ground with suction and discharge flanges visible above the ground.This pump is used to transfer LPG from bullets and is installed outside the bullet not submerged inside the bullet.

The NPSH a at the suction flange may be set as zero at suction flange as NPSH required by machine is fulfilled the height of barrel and extent to which pump is inside the ground.




Regads
SAM

Dear sam,
What I saw& operated at other refinery early days;
a system of LPG Bullets located (resting on concrete saddle supports)
submerged Barrel pump located few meters away with the barrel sump deep in the ground,connecting with pipelines through flanges on either (Suction and discharge) sides.
Pump also utilized with success for LPG Tank trucks emptying in very early days(prior to production at our location)
Hope this helps
Qalander

[joerd]

It helps a lot (probably) if you call these things vertical CAN pumps (or canned pumps) instead of barrel pumps. The confusion arises because for this type of pump the NPSH datum line (reference line) is the suction flange instead of the pump centerline (as for horizontal pumps).
The reference point that matters to the pump is the location of the first stage impeller eye. By extending the pump shaft, and thus effectively increasing the distance from the suction flange to the impeller eye, there is more static head available at the impeller eye.
So the NPSHa per the pump datasheet may be zero, because it is referenced to the suction flange, but effectively, for the first stage impeller there is positive NPSHa.

[djack77494]

Good point, Joerd. All pumps must have available NPSH at their impeller inlet or they will experience cavitation. There are changes in flow streamlines occuring as the liquid moves into the eye of the impeller. These changes require energy which is extracted from the total pressure of the liquid. Thus the liquid must have some pressure (head) available for this work, and we term this as the NPSHa. As this drops off to (and below?) the pump's requirement (NPSHr), the energy still comes from the fluid, only now the pressure drop is such that some of the liquid begins to vaporize. To me it sounds as if the vertical pump manufacturer is employing a "trick" in that he has defined the reference plane (for NPSH calculations) to be at the pump's suction flange and not at the impeller inlet. Despite calling this a "trick", I have no problem with his approach since the reference plane can be arbitrarily defined. It is your responsibility to understand where the reference plane is defined.