30 replies to this topic

[Qalander (Chem)]

Thanks
However, considering that, the head developped by the pump and the head required by the system both include the pressure head terms of source and destination drums, i thought the effect of superimposed pressure would cancel when differentiating the heads. No? I hope my description is clear enough

In a stable pumping system:
Developed head by the pump=(superimposed discharge pressure-superimposed suction pressure)+(velocity head in discharge line-velocity head in suction line)+(static head of discharge line-static head of suction line)+(hydraulic loss in suction line+hydraulic loss in discharge line)

1/ What do you mean by "stable pumping system"?

2/ I rather think what you describe is the system head

1-"stable pumping system"?is a system that could be considered dinamically stable without any considerable change in its main characteristics such as pump RPM,flowrate,pressure heads,....
2-What i described is an equation relates developed head by the pump to system head....

Dear sheiko
I understand fallah in his last post had very correctly/ appreciably put the whole in equation form.
Regards
Qalander

[sheiko]

sheiko,
This may just be a problem with semantics (terminology). When I consider the head developed by the pump, I am speaking only of the mechanical (or hydraulic) abilities of this piece of rotating mechanical equipment. It has nothing to do with any superimposed pressures. When I saw your statement about the head required by the system, I took that to mean pressure differences which were normally or commonly expressed as head (i.e. linear dimension). I see that you are using the term differently and that you are "pulling in" the superimposed pressure terms. Let us not belabor what to call the various terms. My question to you is, "Do you understand and accept the example I previously generated?" If your answer is "yes", then we are in agreement. If not, please provide additional information about what you dispute or don't understand.

My answer is "yes"

But, just to fix ideas about terminology:
. Total system head = total discharge head - total suction head
. Total discharge head = discharge usable head + discharge friction head
. Total suction head = suction usable head - suction friction head
For suction and discharge:
. Usable head = static head + velocity head
. Static head = pressure head + elevation head
. Friction head = equipment/instruments head + straight pipe head + valves/fittings head
. Valves/fittings head = entrances head + exits head + increasers/reducers head + valves head (considering piping valves head and the wide open control valve head) + other fittings head

Now, to me, if i plot the system curve corresponding to the wide open control valve (what i had previously called "head required by the system") versus the pump curve (what i had called "head developped by the pump"), the difference between the two curves will represent the pressure drop available for control, even if the suction or/and discharge pressure head (as defined above) is/are atmospheric or not...Am i correct?

[sheiko]

Dear sheiko
I understand fallah in his last post had very correctly/ appreciably put the whole in equation form.
Regards
Qalander

I agree and i thank Fallah for the help

[Qalander (Chem)]

Dear sheiko
I understand fallah in his last post had very correctly/ appreciably put the whole in equation form.
Regards
Qalander

I agree and i thank Fallah for the help

Dear All
Complete discussion was really learning and informative indeed
Best Regards
Qalander

[djack77494]

2/ I rather think what you describe is the system head

sheiko,
I agree with your assessment. I think fallah is considering that for a pumped system, the output of the pump is equal to the requirements of the system (for a successful design). As I stated in my earlier post, "I am speaking only of the mechanical (or hydraulic) abilities of this piece of rotating mechanical equipment." I went to the effort of stating things this way because I wanted to fully divorce the pump's performance from the system requirements.

In answer to your most recent posting, "am I correct?", I would answer "Yes". But I would do so a bit reluctantly because I think the discussion has gotten a bit muddied. When I do pump calculations, I think of the process as being of two parts. First I analyze the system and it's response to flowrate changes. This essentially defines the system curve. Then I select a pump an verify its compatability with the system curve. You might even think of there being a third part-examination of the separations between the pump and system curves to verify that the control valve will be capable of generating those pressure differences.

There is one point where I do not fully agree with you. You seem to think of the control valve, if fully open, as being negligable. Actually, the pressure drop through a fully open control valve may be and often is appreciable. One way to think of it - the control valve pressure drop consists of the drop when fully open PLUS the additional drop that occurs when it is partially closed.

[sheiko]

There is one point where I do not fully agree with you. You seem to think of the control valve, if fully open, as being negligable. Actually, the pressure drop through a fully open control valve may be and often is appreciable. One way to think of it - the control valve pressure drop consists of the drop when fully open PLUS the additional drop that occurs when it is partially closed.

Thank you for your answer djack77494

In fact, i do not consider that the wide open control valve pressure drop is negligible. I just pointed out wide open , in order to segragate between pressure drop available for control (decreases as flowrate increases) and pressure drop through the wide open control valve (increases as flowrate increases).