3 replies to this topic

[arvind]

Dear all,

I have a little concern on NPSHa calculation for reciprocating pump.
In NPSHa calculation we consider acceleration loss, which is then reduce by installing pulsation dampener.


In the API 14E and ISO 13703:2000 example has given to reduce the pump suction length to 15 x nominal diameter.
I need to know what is the basis for reducing the suction length to 15 x nominal diameter after installing pulsation dampener?

see the below snap shot for your reference from ISO 13703:2000 page 68.

[ankur2061]

Arvind,

The snapshot is missing. Fortunately I do possess ISO 13703:2000. If you refer page 24 / 68, the formula for acceleration head is given as:

h_a = L*v_L*R_p*C / (K*g)

where:

h_a = acceleration head, expressed in meters of liquid
L = length of suction line, m (actual length, not equivalent length)
v_L = average liquid velocity in the suction line, m/s
R_p = pump speed, rpm
C = empirical constant for the type of pump
= 0.066 triplex-single or double-acting
K = liquid compressibility factor, representing the reciprocal of the fraction of the theoretical acceleration head which shall be provided to avoid a noticeable disturbance in the suction piping:
= 2.0 for most hydrocarbons
g = gravitational constant (=9.81 m/s²)

Section 5.3.2.6 - e) says that providing a suction line dampener may reduce L, the length of pipe used in the acceleration head equation, to a value of 5 to 15 nominal pipe diameters.

As I understand by providing a suction line dampener you are reducing the turbulence in the suction line thus reducing the acceleration head value. This has been translated as a reduction in the suction line length and most likely an experimental determination for various types of reciprocating pumps. ISO 13703 does not provide a direct basis for reducing the value of L but as I mentioned earlier this could be an experimental determination with reciprocating pumps.

The example given in page 68 does not consider a suction dampener and thus straight away considers the straight length of the pipe for the value of L in the acceleration head equation.

As a design engineer if I am doing the hydraulic calculations of a reciprocating pump, for a conservative design I would not take credit for suction line dampener and reduce the value of L by 5 to 15 pipe nominal diameters. I would consider the value of L as the straight length of the suction pipe for my acceleration head calculations. This is because Section 5.3.2.6 - e) of ISO 13703 mentions that a good properly located dampener and kept properly charged is required to reduce the value of L. This is a qualitative statement and not quantitative. How do you decide whether the suction dampener is good, properly located and properly charged?

In a nutshell, you may ignore or not take credit for a suction dampener for your acceleration head calculations.

Hope this helps.

Regards,
Ankur.

[arvind]

Thanks Ankur for your prompt response.


Regards,

Arvind

[chemmu]

Dear Sir,

Good Afternoon,

First of all thank you so much for your valuable information on the subject matter.

I have some doubts in acceleration head calculation.

As per my understanding, RPM means - Revolutions Per Minute
SPM means - Strokes Per Minute

the notation RPM which we use for centrifugal pumps and SPM which we use for Reciprocating pumps.
Please correct if i am wrong.

Acceleration head loss we will calculate for reciprocating pumps only.


h_a = L*v_L*R_p*C / (K*g)

where:

h_a = acceleration head, expressed in meters of liquid
L = length of suction line, m (actual length, not equivalent length)
v_L = average liquid velocity in the suction line, m/s
R_p = pump speed, rpm
C = empirical constant for the type of pump
= 0.066 triplex-single or double-acting
K = liquid compressibility factor, representing the reciprocal of the fraction of the theoretical acceleration head which shall be provided to avoid a noticeable disturbance in the suction piping:
= 2.0 for most hydrocarbons
g = gravitational constant (=9.81 m/s²)
But as per the formula, we have to use pump speed in RPM.
For calculating the acceleration head loss for reciprocating pump,shall we use pump speed in SPM in the place of RPM in the formula ?.

will it affect much in the calculation ??

Please give your views on this ...

Thanks in advance..