6 replies to this topic

[kkala]

We intend to perform commissioning tests in numerous new pumps (flow rates 15-800 m3/h), taking suction from tanks, as illustrated in the attached sketch. Water will be used as liquid, or various petroleum products to be handled by the pump in normal operation. So the issue is how much liquid should be contained in each tank.
- Tank liquid level should be enough to result in NPSHa according to pump specification. This level has been estimated.
- But also tank liquid level should be high enough above suction nozzle to avoid air ingression into the suction line and subsequent cavitation. This is the issue, on which advice would be appreciated.

Searching the WWW, I have found necessary submergences for pumps of much higher capacity in http://www.mcnallyin...1-html/1-3.html, or for vertical pumps (as understood) in http://www.pumps.org...il.aspx?id=4012, but not for horizontal centrifugal pumps in the flow range of 15-800 m3/h. Hydraulic Institude HI 9.8 standard may be useful, but is not free.

Attached Files

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[breizh]

Hi Kostas ,
Take a look at this link:

http://www.gouldspum...m/pag_0006.html

Breizh

[kkala]

Thank you, Breizh, the submergence (H) - suction velocity diagram from Goulds Piping design Section 02 (specified in the link) clarifies my question.
It is pointed out that submergence H is considered from centerline of the suction.

Trying to interprete the variation of H indicated for different flowrates (200-5000 GPM), I have considered a suction velocity of 5 ft/s (same for all cases) and a suction size of 4", 6", 10" connected to an horizontal centrifugal pump. The cases correspond to 198 GPM, 450 GPM, 1229 GPM; diagram indicates H = 2.8, 2.9, 3.1 ft respectively, that is 31.6 in, 31.8 in, 32.2 in over the upper part of the tank outlet nozzle. For constant suction velocity, required submergence (H) seems to slighly increase with suction diameter, even if H is considered as liquid depth to the upper part of suction nozzle.

Above also indicates that required submergence cannot be expressed as e.g. X suction diameters.

[chemsac2]

kkala,

November 2010 issue of Chemical Engineering magazine (www.che.com) had an article "Predict and prevent air entrainment in draining tanks" which gave correlations required for minimum submergence.

It is also kind of review of literature published on the topic till date.

Regards,

Sachin

[sheiko]

See also: http://webwormcpt.bl...e-to-avoid.html

[kkala]

Thank you, Sachin and sheiko, for your useful contributions. I will search Chemical Engineering of Nov 2010 for the article of submergence. Reference by sheiko was found in eng-tips, but not explained clearly enough. Now it is clear and with an arithmetic example.
By the way, latter formula can be transformed as (s-d)*d^1.5*g^0.5/Q=2.93, which is independent of the system of units. g is gravity acceleration =9.81 m/s2 and rest symbols as in the reference.
The arithmetic example gives s=0.66 m, while s~2.4 ft (0.73 m) per Goulds Pumps diagram, referenced by Breizh(1000 GPM - 2.83 ft/s). But GouldsPumps consider distance from suction centerline, so 0.66 m should be compared to 0.58 m (above upper end of nozzle in both methods), which is a fairly good agreement.

Edited by kkala, 08 March 2011 - 03:54 PM.

[kkala]

Some additional remarks on the formula referred by Sheiko may be useful.
1. Arithmetic example seems to consider Q/d instead of Q/d^1.5 (apparent error), but the result of s=0.66 m is according to the formula.
2. Considering Q=(π*d^2/4)*V, where V=(average) suction velocity, mentioned formula can be simplified to
s = d + 2.3*V*SQRT(d/g), where
s=submergence required over upper part of suction nozzle
d=suction nozzle diameter
g=gravity acceleration