7 replies to this topic

[a1986]

Hi Everybody,
i have some doubts about a tender for vertical barrel pump (10 stages) for LPG. I list them below:
1. NPSHa is 0 m and NPSHr is 2 m.. i have thought that i shall calculate effective NPSHa referring to distance from 1 stage impeller and discharge flange (at the same axis of suction);
2. i have pump performance curve too..and i have noticed that operating point is at 166 m^3/h of capacity, but curve finish at 177 m^3/h of capacity..it is a very strong situation, and i'm afraid. in accordance with api 610 this pump is undersized and shall be rejected.
i would your opinion about this situation, and i would know the consequences of the choise of this pump...i try to explain better: i would know what appened if operating point is so close to curve run out?(maybe oveloading of electric motor?)...when i'm examining an LPG pump, i should examinate what in particular of LPG properties?

best regards

[Art Montemayor]

A1986:

You are not communicating well. If you are evaluating a specific pump for an application, you should furnish at least two (2) basic engineering documents:

• A detailed pump Data Sheet;
• The pump’s performance curve.

Without access to these documents, it is difficult for the Forum members to relate to what you are stating. Your basic statement saying that NPSHa = zero meters can’t be true if the application is a real one. You MUST supply the required pump’s NPSHr PLUS approximately an additional 35%. Otherwise, the pump will be rejected for the simple reason that it won’t pump.

What section of API 610 are you referring to? Please cite the specific section.

[a1986]

ok i try to explain better..
1. a barrel pump is used when there is npsh problem..in this case i have npsha=0 m and npshr=2 m...i can value effective npsha calculating the distance between first stage impeller and discharge line..in this case i have npsha= 4 m
2. operating point of that pump is very next bottom curve, and api 610 express that from best efficiency point must be a range of 80% and 120% to individuate allowable region...(see attacched file)
i think that i could have some problems installing this pump..what kind of problems could i have?referring to motor?

thanks

[TS1979]

1. a barrel pump is used when there is npsh problem..in this case i have npsha=0 m and npshr=2 m...i can value effective npsha calculating the distance between first stage impeller and discharge line..in this case i have npsha= 4 m.

The point 1 still does not make sense. If the NPSHa=0m and NPSHr=2, the pump will simply not work and cavitation will happen to the pump. The NPSH calculation is always from the storage tank (if you have one) to the centerline of the pump suction regardless to the pump discharge line.

2. operating point of that pump is very next bottom curve, and api 610 express that from best efficiency point must be a range of 80% and 120% to individuate allowable region...(see attacched file).

First, there is no file attached in your post.
Second, the best efficiency point is given by the pump manufacture. According to API610, as long as your operating point is within the capacity range of 80 ~ 120 % of best efficiency point, you are OK. However, to satisfy the capacity requirement, you need to be careful to decide what is your required capacity.

[narendrasony]

a1986,
Barrel pump is nothing but a vertical canned pump with first stage impeller at the bottom of the submerged can / case. As I've seen, only suction and discharge flanges are visible at ground level and the can is underground below these flanges.

With reference to the suction flange NPSHa may appear to be very less or zero / less than NPSHr ; but with reference to the first stage impeller eye, it is increased by the distance between suction flange and 1st stage impeller eye or the submergence. Hence essentially there is positive NPSHa at impeller eye.


Regards
Narendra

[a1986]

http://imageshack.us...141/imgdqw.png/ this is the example of operating point of the barrel pump..
i think this pump could have several problems...what do you do in this situation, watching an operating point like this?

a vertical suspended barrel pump is a vertical turbine too..suction and discharge flange are at the same axis.and the legnht of barrell is 4 m...how much do lpg properties influence pump running?

[a1986]

if i would verify npsha of a barrel pump like this, how can i do?
i know lenght of can in which is installed, suction and discharge pressure, and fluid properties...
help me please!

[kkala]

Seeing that thread has been stagnant, following notes could be useful (comments welcomed),
1. narendrasony has given explanations on the referred vertical barrel pump, also called encased vertical pump (burried). I have seen them draw LPG / anhydrous NH3 from bullets / spheres, where NPSHa is quite low. These pumps have low NPSHr referring to suction nozzle, that is if center of inlet flange is considered as datum for the NPSHa. The diagram given by a1986 (post 6) indicates approximately NPSHr~18ft = 5.5 m of liquid, assumed to refer to the entrance of the first impeller met by the moving liquid.
2. Mentioned NPSHa = 0 m evidently refers to pump suction nozzle. NPSHr referring to suction nozzle has to be lower than this.
If length of (mostly burried) barrel is 4 m, NPSHr referring to suction nozzle could roughly be 5.5 - 4 =1.5 m, plus relevant pressure drop along the barrel. But Vendor can supply precise figure for the operating point, it is probably mentioned 2 m (not complying with NPSHa=0 m).
3. Pump operates too close to the end of curve point, so it is undersized. There is not a reason for a new pump to be so.
Besides, will operating point on the steeply descenting part of the curve (head vs flow) cause unstable operation? as if I read so in the past, advice welcomed. Pumps operating close to end of curve (undersized) have not been seen, contrary to pumps operating close to shutoff (oversized, not a recommended case but seen).
Nevertheless, at least here, main assessing of pump curve is a task of Machinery Dept, not of Process.
4. Pump must have antispark protection, since LPG gas leakages will create a hazardous area there (if there is not by other source).
NPSHr reduction due to hydrocarbons should not be taken into account.
LPG properties will be used to estimate ΔP in the suction line (for the NPSHa). Vapor pressure is actually not used, considered same at storage and pump suction.
5. http://www.cheresources.com/invision/topic/15633-horizontal-vs-vertical-pump-speeds and its internal references could be a bit useful.

Edited by kkala, 20 October 2012 - 04:56 AM.