Throttling pump to get it back on the curve
Throttling pump to get it back on the curve
(OP)
Hi all,
I have a situation where the pumps are pumping outside of its curve, max flow.
the client has no problem with throttling the outlet to move the duty point back on the pump curve, he knows all about the energy losses and don't care. I was concerned about the pressure drop required to reach the required duty point, I'll need approximately 3.4bar drop.
We will get this by semi closing a butterfly valve. Will this pressure drop cause cavitations? Even if it does and the only thing that will get damaged is the valve and a piece of pipe then no problem. Does this valve need to be away from the pump “few meters” to prevent cavities from affecting the pump it self?
Is there something else I should worry about?
Regards.
I have a situation where the pumps are pumping outside of its curve, max flow.
the client has no problem with throttling the outlet to move the duty point back on the pump curve, he knows all about the energy losses and don't care. I was concerned about the pressure drop required to reach the required duty point, I'll need approximately 3.4bar drop.
We will get this by semi closing a butterfly valve. Will this pressure drop cause cavitations? Even if it does and the only thing that will get damaged is the valve and a piece of pipe then no problem. Does this valve need to be away from the pump “few meters” to prevent cavities from affecting the pump it self?
Is there something else I should worry about?
Regards.





RE: Throttling pump to get it back on the curve
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Throttling pump to get it back on the curve
If cavitation is a problem then you can install several plates in series.
Whether or not cavitation is a problem will depend on the pressure at the pump delivery.
RE: Throttling pump to get it back on the curve
RE: Throttling pump to get it back on the curve
RE: Throttling pump to get it back on the curve
RE: Throttling pump to get it back on the curve
During commissioning it is not uncommon to find this situation where you have an over-sized pump. Typically you would overcome this for the first few hours by throttling the valve, then for the next few days by inserting a properly sized restriction orifice. Once the system has settled down and you have the final flowrates and pressure drops you would implement a permanent and economic solution - either install a VSD as recommended by Keith, or get the impeller trimmed to the correct diameter.
BRIS - how did you get from 3.4 bar to 25 m/sec?
Harvey
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Throttling pump to get it back on the curve
The 25m/sec is simply the velocity to be destroyed to create a head loss of 34m. orifice loss = (Vvc-Vds)^2/2g (for water) Vvc = velocity at vena contracta, Vds = velocity in downstream pipe.
RE: Throttling pump to get it back on the curve
No point using a VFD if you're not too concerned with energy loss, they'll add more maintenance headaches.
Orifice plates usually do not have enough variability if the flow changes later, but could be a quick solution, esp if the flow doesn't vary much.
Throttling is likely to control cavitation if cavitation is an existing problem. It won't add to that problem.
RE: Throttling pump to get it back on the curve
RE: Throttling pump to get it back on the curve
The noise, lack of variablity, and excess backpressure from a control valve + orifice plate(s) required by a too-large pump are incredibly short-sighted solution by management. VFD is a much better solution in the long run....
RE: Throttling pump to get it back on the curve
Throttling the pump will use less power than the pump is currently using- but as per above- what effect does the reduction in flow have on your process?
RE: Throttling pump to get it back on the curve
What is the flow and unthrottled head and what is the required head of this pump.
Naresuan University
Phitsanulok
Thailand
RE: Throttling pump to get it back on the curve
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Throttling pump to get it back on the curve
these are existing pumps, throttling will be for a short period of time. its pumping 300 l/s against 72m head through 600mm GRP pipeline.
control valve and VSD are more expensive in capital cost and will take long time to be done. the client wants something low in capital (high in O&M is acceptable due to budget allocation) as long as it solves the problem and can be done fast.
The orifice plate seems to be the best solution, I've never done any, I'll Google it and see what comes up, in the mean time can anybody point me in the right direction on how to size these , installation , cost ? is not possible to get these on a rotating disk ? ( so you can increase or decrease the area )
these pumps are not pumping outside of their curve now, but will be ,because the client is looking to add a new connection to the existing main. The idea is to throttle the pump some where (maybe at the connection point) to pump exactly the same flow it is pumping now (not more) by splitting the flow into the two mains.
trimming the impeller is a good idea but cannot be done cause the operator might wish to turn off the new connection and pump through the old one only.
thanks all for your post
RE: Throttling pump to get it back on the curve
In your case I would recommend two orifice plates - (to reduce noise) construction is simply a plate with a hole placed between flanges on the pipeline. Head loss across the orifice is H = (Vcc - Vds)^2/(2g) (Vcc = velocity at the vena contracta. You can take the area at the vena contracta to be about 0.65 x the area of the hole in the orifice. Vds = the velocity in the downstream pipe. (note most calculations are based on using orifice plates as a flow measuring device and are not relevant to your application).
I have never considered a variable orifice. It is a simple procedure to swap plates when you want to change system characteristics. If you oversize slightly you can throttle back (fine tune) with your butterfly valve.
RE: Throttling pump to get it back on the curve
In a pipe this diameter you will need to check the mechanical strength of the orifice plate. I have seen 1/8 inch plate used for orifices that later looked liked they had been deliberately dished. A worst case would be for the orifice to actually tear and have a chunk of steel going into some downstream piece of equipment. Get a competent mechanical engineer to check the plate thickness required.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Throttling pump to get it back on the curve
Using orifice/s plates to restrict flow is common usage for controlling power input to a pump or to control discharge and seems a logical way for you to go.
Naresuan University
Phitsanulok
Thailand
RE: Throttling pump to get it back on the curve
If the short duration still is months/year yu should consider the trimming of the impellar (same relation in flow/head/power) vs impellar size as for VFD. But then you must order a new impellar to replace the one thats been altered.
Remember that 3.4 bar at a flow rate of .3 m3/sec corersponds to a power consumption of approx 100 kW (with an assumed 100% efficiency!).
Best regards
Morten
RE: Throttling pump to get it back on the curve
Naresuan University
Phitsanulok
Thailand
RE: Throttling pump to get it back on the curve
$0.15/kWhr x 2,880kWhrs = $432/day = $158,000/year
Yeah just slam in an orifice plate, they're cheap!
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com