Pump Mini Flow ? 1

[brochon]

Can anyone help me determine pump mini flow requirements.

I would like to verify the pump vendors recommendation of 60% of rated flow. The pump manufacture is basing this not only on protection the pump under closed discharge conditions but also for radial & axial thrust affecting mechanical and hydro-dynamical pump components. Mechanical seal and bearings are the most affected elements.

Any comments on mini flow requirements for axial & radial thrust?

Thx

 

[pmover]

in short, the pump mfg. established criteria for minimum flow conditions. the designer is responsible to apply the means to ensure pump operates within its design limitations (max and min flows).
briefly, the statements you provided regarding thrust loads are applicable to certain pumps. again, the pump mfg needs to consider these loads in the design of their pumps.
-pmover

 

[Kawartha]

brochon,

As indicated in pmover's email, you must rely on the pump manufacturer for minimum flow rates. Minimum flow rates can vary from 25 to over 60% of bep flow depending on the pump design. The pump warranty will depend on you operating within the acceptable operating envelope that the manufacturer provides. You should obtain this information in writing from the manufacturer.

To obtain another opinion from this thread, please provide additional details for the pump. Pump type, capacity and head, driver details, liquid details, liquid temperature, pump specific speed, pump suction specific speed, etc.

 

[BobPE]

the typical engineering operation envelope for most centrifugal pumps is 60% to 120%. Like was said in previous posts, it depends on your pump and what you are pumping. The forces that can be imposed by operating outside of this envelope are very real and typically result in unbalanced thrust causing radial and axial impeller loading. The result can be cavitation, impellers broken at the shaft, pump column failure, seal bearing and seat wear and failure and vibration among other things.

Please tell us more if you can.

BobPE

 

[d23]

brochon

There is some very good post so far. As a manufacture I would like to add a couple thoughts.

1) You use the terms “Supplier” and “Manufacture” in your post. I would recommend that you forget the supplier. The manufacture is the only source that can answer your question with any accuracy.

2) I would guess that you are trying to operate over a very broad operating range by choking or restricting flow. As a manufacture I would not mind operating a pump at 10% of its BEP provided I know about it ahead of time. There are several design issues that must be addressed before doing this, but it can be done without sacrificing run life.

Good Luck!

 

[quark]

d23 is right. Recently I have installed atleast 20 pumps with manufacturer's specified minimum flow rate of around 10%. There was one good post, sometime back, about this very question. TD2K gave one good example with respect to the acceptable temperature rise. (and pump efficieny is the main variable)

And I don't think any problem with radial thrust at reduced flow condition.

 

[Scipio]

I agree with d23 & quark in the last couple posts, flows as low as 10% are achievable, but, as d23 points out from his perspective as a manufacturer, they have to know you intend to operate it there.

I have to disagree with quark, though, in that radial loading at reduced flow can be a problem. Pumps develop radial thrust even at BEP, though obviously it's higher in high-head pumps with larger impellers, they're just built to cope with it. Lowest radial thrust is at BEP, highest usually at shutoff, just bear in mind that it's not the reduced capacity that increases radial loading, but the higher heads associated with reduced capacity. If a pump is intended to be operated at reduced capacity for prolonged periods of time, they can be built with heavier shafts & bearings to handle the load, or be built with double volutes to reduce the load the shaft & bearings have to handle.

Like a lot of things, it is a problem, but it's not one that can't be addressed by throwing more money at it. Karassik's Pump Handbook actually does a pretty thorough job of covering radial loading.

 

[TD2K]

There was a recent article on minimum flow in "pumps and systems" magazine which recommended the minimum flow as the flow corresponding to 98% of shut-off head.

We are just in the process of installing several pumps, I'll go see how this correlation seems to agree with the vendor recommendations.

Minimum flow has definitely increased over the years. On this project, when I developed the preliminary data for the minimum recir valves, I used 25% and 40% of design flow to size the recirculation lines and control valves. On several of them, I had to subsequently bump up the flow when we got vendor data. Not enough to change lines sizes or control valve sizes, just update the paperwork, but I had thought I had bracketed the final numbers pretty well.

 

[brochon]

I thank all of you for your replies. Some of you have asked for additional information. Hopefully, this will help with my problem.

An existing pump was not operating very well (i.e. cavitation, frequent seal replacement etc.). The pump was originally sized for a flow rate of 3300 gpm, with no mini flow line. Normal operation is ~1800 gpm. Our pump problems were attributed to an over sized pump, with low normal flow.

To fix the problem, the existing pump was replaced with a smaller pump. No mini flow line was installed at this time, as we were retrofitting on the run. The vendor has recommended a mini flow of 1500 gpm. At times, the process does demand less than 1500 gpm. We are concerned when running below 1500 gpm, we may experience similar problems as in the past (i.e. cavitation & frequent seal replacement).

I am thinking the vendor may be being a little conservative and operating some what below 1500 gpm some of the time will not cause us grief. I am hoping some of you can help!

Here are some details on the new pump:

Make: Union Size & Model 8 x 10 x 13 HOL
Centrifugal pump, double volute
Capacity: 2450 gpm Head: 555 ft
Driver Details: 650 hp
3550 rpm
4000 v

Liquid Details: Fluid: HVGO @ bubble point
Temperature: 550 °F
Vapour Pressure: 0.15 psia
Specific Gravity: 0.72
Viscosity: 0.25 cP
Specific Heat: 0.69 Btu/lb °F

Specific Speed: 1540
Suction Specific Speed: 9550

Thx Brian

 

[TD2K]

Brian, I have a 'generic' minimum flow calculation method though of course, it has the expected caveat that you need to confirm the numbers with the vendor.

Assuming this is a single stage pump, I get about 29% of the BEP flow with this method which seems to be a lot less than what the vendor is telling you (if you have 2 stages, the minimum flow is even less).

However, it's their pump. I'd talk to them to see how often and how much under 1500 gpm you expect to run and go from there. Looking at the pump curve, does the NPSHR increase significantly at the possible minimum rates you might run?

 

[gregcase]

There is alot of good advise in the previous posts. I'm going to wirte this as a previous chief engineer for a couple of pump companies and currently as a design consultant. The manufacturer decides what the minimum flow for the pump is. Guidelines such as the Hydraulic Institute Standards give Manufacturers a variety of factors to consider when setting the minimum flow. These are:

1) Temperature build up
2) Excessive radial and axial thrust
3) Suction Recirculation
4) Discharge Recirculation
5) Insufficient NPSHA
6) Pump Energy and suction energy level
7) Resrictions due to materials
8) cooling loop problems to canned motors mag drive and seals.

My point is, the user can calculate the their own Minimum Flow but the Mfg can still deny any warranty claims based on improper operation of the pump. You will find that operation out of the preferred operating range will cause a multitude of maintenance problems. See: "Operating at a Pumps BEP", Chemical Processing Magazine, December 2001. The question should not be can I operate at min flow below what the mfg recommends. the question is what can I do to improve the operating point of the pump system. Consider using a VFD to reduce the pump speed instead of throttling the flow. You will achive the lower flow rate (meet your process goals), the hydraulic forces in the pump will reduce with the square of the speed change (lower maintenance costs), and you will reduce the power consumption of the pump (reduced energy bills).

 

[Kawartha]

brochon,

Based on the details which you provided Apr. 21st, your pump is considered as a high suction energy pump (Hydraulic Institute 9.6.3-1997). Because of this your minimum flow rate is very important because suction recirculation is possible at lower flow rates.

For additional information, the Hydraulic Institute recommends a minimum flow rate of about 45% of bep, to prevent suction recirculation. There are also many other concerns regarding minimum flow - Note above post by gregcase.

Try to make sure that you do not operate below the manufacturers stated minimum flow rates (on a continuous basis), except for short periods of time. Keep in mind that the manufacturer bases his information on many pumps in service, and is also probably also being very conservative.