Pump Throttling vs. Restriction Orifice... 2

[AJC9725]

I'm having a hard time understanding the comparison between pump throttling and orifice restrictions. What I mean by this is:

1.) When throttling the discharge of a pump, you are restricting flow. As you restrict this flow, you are INCREASING the pressure head. This is evidenced by moving left on any pump curve.

2.) If you are using a restriction orifice, you are desiring to limit the downstream flow. Downstream of the orifice, the flow is less, and the pressure also DECREASES as compared to upstream of the orifice.

This is where my confusion comes in. In both scenarios above, you are restricting flow...in one case you are restricting flow downstream of the throttled discharge valve, and in the other case you are restricting flow downstream of the orifice. Why then, in the pump throttling scenario does pressure INCREASE because of this restriction...while in the second case, pressure is DECREASED downstream of the orifice?

Thanks to anyone who can help!

 

[Pumpsonly]

The difference is:
In the 1st scenerio you are measuring the pressure before / up stream of the valve.
In the 2nd scenerio you are measuring the pressure after/down stream of the valve.

If you measure the pressure at the same location, both will shown the same effect.

 

[MikeHalloran]

The two scenarios you describe are the same scenario; just one.

Think of the valve as being a variable orifice.

When you close the valve a little, or introduce an orifice,
the pressure upstream of the orifice or valve tends to go up;
that is also the pressure that the pump sees, and that in part determines the pump's operating point.
... and the pressure downstream of the orifice or valve falls,
because of the pressure drop across the orifice.



Mike Halloran
Pembroke Pines, FL, USA

 

[bimr]

They do exactly the same as the previous posters state. The difference is that there is more flexibility when a throttling valve is used.

 

[BigInch]

An orifice can work well, if you only run at one flowrate. With a wide range of flowrates an orifice is unlikely to give you the control you'll need.

Independent events are seldomly independent.

 

[SNORGY]

AJC9725:

With respect to your statement at 2):

"Downstream of the orifice, the flow is less, and the pressure also DECREASES as compared to upstream of the orifice."

The flow will be the same on both sides of the orifice, both in terms of mass flow rate and volumetric flow rate (for incompressible liquid).

 

[AJC9725]

SNORGY...wait...now I am a little confused, after reading your last post.

We agreed earlier that throttling a pump, and a restriction orifice both achieve the same thing...reducing flow on the downstream side. This is evidenced by moving left along the x-axis on a pump performance curve.

I have also seen restriction orifices at my plant, where a main cooling water line branches off to a line that provides cooling to a mechanical pump seal. There is an orifice in this line, and I thought its purpose was to RESTRICT/REDUCE the flow to the seal from the main cooling water line.

But in your above post you mentioned that flow on both sides of an orifice are THE SAME.

Can you explain what I am missing?

Thanks.

 

[SNORGY]

AJC9725,

The pump moves left on its curve because the orifice adds head loss to the system head curve. A centrifugal pump will only act at one single point along the locus of points lying on its head-capacity curve, and that point is where it's head capacity curve intersects the system head curve. So, you are not controlling the pump simply by reducing the flow rate downstream of the orifice; you are increasing the system resistance to the flow, and the flow is the same throughout: downstream of the orifice, upstream of the orifice, discharge of the pump, suction of the pump....

It must be so in order to satisfy continuity.

 

[Pumpsonly]

I have also seen restriction orifices at my plant, where a main cooling water line branches off to a line that provides cooling to a mechanical pump seal. There is an orifice in this line, and I thought its purpose was to RESTRICT/REDUCE the flow to the seal from the main cooling water line.

AJC 9725, you are getting yourself mixed up here. You are talking about a branch line in the system.
The flow rate before(up stream) and after(down stream) the RO in that branch line must be the same.