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Operating at extreme left end of Pump Curve

Operating at extreme left end of Pump Curve

Operating at extreme left end of Pump Curve

(OP)
I have a Centrifugal Pump designed for about 12m3/hr (from purchase information with the supplier). The pump curve is without efficiency lines and as follows:



We are however, running the pump at only 3.5 m3/hr maximum, which based on the above, means it's not even on the curve. Although I don't have the BEP, but based on the rated 12m3/hr, this means it's about only 30% or less.

This is obviously not ideal and may lead to chronic failures of parts like seals, bearing, etc. The seal has actually failed twice (leak) in the last 2 years. Apart from changing to a new pump altogether, what are the cheaper options to solve this?

a) Install a recirculation line at the pump to increase the flow at the pump?  Comments?

b) Reduce the pump speed by installing a variable speed drive??  Comments??

c) What else?

---engineering your life---

RE: Operating at extreme left end of Pump Curve

How did you check the flowrate? Do you have a flow meter installed? Alternately, did you check it with respect to power consumption? Presuming that flat curved pump is selected for a specific purpose, for ex. high static head application, I don't suggest VSDs.

The absence of flowrates below 4 cu.mtr/hr, from the performance curve, may indicate that the safe minimum flow is 4 cu.mtr/hr.

The ceapest option would be to change the impeller by a lower diameter one.

A seal leaking twice in two years is not uncommon to me though others may object.

RE: Operating at extreme left end of Pump Curve

(OP)
quark, there's a flowmeter installed and in fact, the flow is controlled to a maximum of 3.5m3/hr on the pump discharge side.

I've not completed the review of the head required, but yes, it appears that this pump is for a high static head application. Minimum looks like 20m of head, not yet counting the friction losses.

By the way, just from my info alone, would you immediately conclude that running in the range that is absent on the curve (left hand side) is the main cause of the seal failures?

---engineering your life---

RE: Operating at extreme left end of Pump Curve

No, that is a guess work. If your drawing is a copy of the original curve given by the manufacturer, there is no point in omitting the flowrates below 4 cu.mtr/hr unless it is not advised to run the pump below that. Further, 3.5/12 = 29%, which is close to generally recommended minimum safe flowrate.

The seal leakage may be due to 3 reasons.

1. As you are operating the pump at very low flowrate than
BEP, there may be heating up and this may be causing damage.

2. You may have unbalanced seal. A flat curved pump may not cause damage to the seal but there are some chances.

3. Seal type may not be appropriate for your service. If your application is clean water, then strike out this option.

Where is the seal failure exactly? From the primary mating surface or secondary sealing?

RE: Operating at extreme left end of Pump Curve

How is the pump controlled to 3.5 m3/hr?
knowing an inlet and discharge pressure would also help.
what is the application?

Naresuan University
Phitsanulok
Thailand

RE: Operating at extreme left end of Pump Curve


As an example, when the pump is intended to supply liquid to spraying nozzles of a battery of machines, the flow rate will vary with the number of machines. On the other hand, the head would have to be kept constant over the working range of the pump to get best results. In such a case a flat Q-H curve would be the preferred choice.

Answering quark's and Artisi's questions would be a good start.

RE: Operating at extreme left end of Pump Curve

(OP)
Artisi, as I mentioned earlier, there's a flowmeter and control valve installed on the pump discharge side. The flow is controlled to a maximum of 3.5m3/hr. Application is for oils with normal viscosities, less than 25cP. At the moment, there are no gauges installed on the suction and discharge sides. But we will be installing them soon and will then inspect the pressure conditions.

Quark, your points have also been considered, namely quality (suitability) and installation of seals. However, the very low flow is something that stood out, in my view;  thus I was wondering if everyone else here would have just made the same automatic conclusion without further work?

---engineering your life---

RE: Operating at extreme left end of Pump Curve


Ddkm you may be right.

Axial and radial thrusts set up in single volute pumps operating under part loads for protracted periods may require suitable and reinforced bearings.

Worn out bearings are one important possible cause for a short life of the seals.

RE: Operating at extreme left end of Pump Curve

Sustained ops under 40% is absolutely begging for trouble.  Its just a matter of when it comes.  60% minimum is suggested by API.  If your 3.5 m3/hr is a steady flowrate, you need to change the pump.  The use of VSDs is not specially cost effective unless flow varies between 40% minimum and 100%-110% and probaby not even work if static head resistance is more or less constant and remains high at low flowrates.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Operating at extreme left end of Pump Curve

(OP)
BigInch,

If VSD's not suitable, what about installing a Recirculation line at the pump itself which will increase the total flow through the pump? Any comments about how easy to do this?

---engineering your life---

RE: Operating at extreme left end of Pump Curve

The only real disadvantage to that is the product may be subject to heat up due to the continuous recirculation at low flow rates.  Recirc at normal flowrates just costs money and usually won't generate much heat.  If you must flow very lo for short time periods where heat is a problem, you might be able to provide some additional above ground pipe length to dispose of some of the heat before it gets back to the pump in order to keep operating for a longer time until temp finally gets hi.  I have had to recirculate all the way back to the supply tank in one or two situations to mix the heat up with the tank contents.  But anyway something like that would probably work fine.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Operating at extreme left end of Pump Curve

This particular problem hinges around a pump with a rating of around 3 - 4kW so the economics seem to dictate that fitting a pump that is a better selection for the application is not beyond the realms of possibility.

the flow rate is known, but not the head, which is necessary to make an informed decision on either, how to overcome the problem or to selected a more suitable pump.

To me - pump hydraulics are pump hydraulics as is good design, if this were a 3000kW unit, I doubt you would be happy running it at such a massive turndown in output and expect it the be trouble free.

Calculate or measure the total head of the system (excluding the head loss acros the control valve) and purchase some thing like an inline Grundfos circulator pump rated for 3.5m3/hr at the required total head and sit back and forget about your problems.

Naresuan University
Phitsanulok
Thailand

RE: Operating at extreme left end of Pump Curve

Artisi,  How's the situation?  You're OK I guess.  Business as usual?

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Operating at extreme left end of Pump Curve

Actually there doesn't seem to be any problems here at all, appears the "thinking" people, although nervous,  are supportive of this action. Just one more step in the country's development.

Naresuan University
Phitsanulok
Thailand

RE: Operating at extreme left end of Pump Curve

As usual, there must be some underlying "history" that I am not aware of.  I see "somebody" has already called for an end to this and a "return to democracy".  Well then, just to wish everything works out peacefully.  Take care.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Operating at extreme left end of Pump Curve

Nice to know that.

RE: Operating at extreme left end of Pump Curve

(OP)
Artisi, you are right about the pump power. It's rated at 4.4kW. However, I have left out an important point earlier. For most of the time, this pump runs at below 3.5m3/hr, but around once every two days, it is required (preferred) to run 8-10m3/hr for a short period of about 1 hour. (For a different product run).

In that case, I'm leaning closer towards a recirculation line at the pump, which I can then close off whenever I want the pump to run higher flows. Right?

BigInch, I think the heat built-up you mentioned (due to low recirculation rates) should not be an issue. After all, with Recirculation, we are actually trying to increase the flow at the pump, while keeping the net flow at the discharge END to the minimum. Right?

Assuming Recirculation is the preferred solution, then what are the considerations when sizing the new recirc line at the pump? Obviously flow is the main consideration - we may want the recirc line (blue line) to run about 8.5m3/hr, so that the total at the pump becomes 8.5+3.5 = 12m3/hr, as per the original design of the pump.



What do you think?

What other considerations besides flow?



Regards,
DD

---engineering your life---


RE: Operating at extreme left end of Pump Curve

thanks

Naresuan University
Phitsanulok
Thailand

RE: Operating at extreme left end of Pump Curve

The solution as proposed works, obviously just a bit energy inefficient.  Fortunately a rather low power application so, perhaps costing you around $3500/yr.  VFD may be feasible, especially if the other product volume requirement becomes a larger percentage of the pump's load someday in the near future.  Recirc at 70% BEP appears to be the best compromise available today.  It is greatly preferable to running at 30%.  Keep the VFD idea in mind.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Operating at extreme left end of Pump Curve

Recirculating the discharge straight into suction is something I avoid. The pumped fluid will get heated up continuously. Secondly, even with 12.5 cu.mtr/hr and 40 mtr head, the power consumption should be around 2 kW (pump eff. is 0.7 and motor efficiency is 0.9, considered).

Why not a smaller second pump that is running at BEP?

RE: Operating at extreme left end of Pump Curve

I second Quark.  If heat is a problem why not run two pumps in parallel so that one (a smaller pump) can be used for the lower flow <3.5 (m^3/hr)applications and both can be run together for higher flow applications without running at low efficiencies.

Kyle

RE: Operating at extreme left end of Pump Curve

(OP)
To update this:

I ran the pump yesterday and found that the pressure at the pump discharge (local gauge) showed only about 1.3bar which equates to about 15m Head (based on the fluid density of 900kg/m3). Based on the Total Static Head plus all the Friction Losses in the rather long pipeline, my initial calculation was about 16m for the Total Developed Head. That means both numbers are pretty equal.

To me, this is surprising if we expect the pump to be oversized and currently running on the extreme left end of the Pump Curve. Shouldn't we expect to see a far higher Head in this case? (As per the pump curve in the first post).

Is there something wrong with the analysis?



Regards,
DD

---engineering your life---


RE: Operating at extreme left end of Pump Curve

Where is the gauge installed? Before or after the valve?

If before the gauge, then, is the pump hunting when you are running it at the part load condition? I read in some past threads that manufacturers deliberately chop off the unstable region of the performance curve. I doubt whether this is the case with a flat curved pump. If you find hunting, then this may also be the reason for seal failure due to the axial movements of the shaft.

RE: Operating at extreme left end of Pump Curve

same question as quark, where is the gauge fitted plus - what duty did you run at --  3.5 or 8 10 m3/hr

Also the curve it is drawn on log / log paper so the curve, although flat is not really as flat as it looks.

Check with Grundfos pumps - they make a number of 2 and 3 speed pumps - I'm not sure in what size or capacities - but you might find something to fit your duties.

Naresuan University
Phitsanulok
Thailand

RE: Operating at extreme left end of Pump Curve

(OP)
The gauge is at the pump discharge but before the throttling valve controlling the flow to a max of 3.5m3/hr.

Refer this diagram:
http://i76.photobucket.com/albums/j24/dd_km/Gauge.jpg

Quark, could you elaborate more on the hunting part? Unless you mean that the curve becomes a Steep Drooping curve at Q below 4m3/hr (which we can't see from the manufacturer's curve above).



Regards,
DD

---engineering your life---


RE: Operating at extreme left end of Pump Curve

What has the pump manufacturer had to say about running at 3.5m3/hr ??? - I'm sure they would caution against it as the pump is probably very unstable at this point.

Naresuan University
Phitsanulok
Thailand

RE: Operating at extreme left end of Pump Curve

(OP)
We managed to contact the pump manufacturer today (it's a darn old pump...), and based on their reply, the minimum allowable flow is actually 7m3/hr for the heads of 30-40m (as per design).

OK, that much is clear, i.e. we shouldn't be running at the low flowrates.

What I still don't understand is this:  
1) If we are truly operating at the extreme left end of the pump curve, how come I don't see the high head of 30-40m on the pressure gauge at the discharge line???

2) Why is the pump designed for 30-40m of Head in the first place, when currently my estimate shows we only require a Total Developed Head of 16m?

Can anyone offer some clues of where I'm going wrong with this analysis????



Regards,
DD

---engineering your life---


RE: Operating at extreme left end of Pump Curve

Is the impeller still full diameter?

Naresuan University
Phitsanulok
Thailand

RE: Operating at extreme left end of Pump Curve

what is the condition of the impeller,  does it have impeller and / or casing wear rings - is it open or closed impeller.

As you can see, there is a number of things were don't know or understand and as we don't have a crystal-ball it's like trying to answer the old story - how long is a piece of string?  

Naresuan University
Phitsanulok
Thailand

RE: Operating at extreme left end of Pump Curve

There is definitely a problem with your gauge reading, may be a faulty gauge. The gauge reading should be about 2.3 bar even when the pump is not running, due to the suction static head. So, in running condition, this should be minimum 2.3 bar and should be higher if the pump is running OK.

We would like to get some more data.

1. Is the direction of rotation of pump OK?
2. As Artisi suggested, what is the condition of the impeller?
3. What is the power consumption at the two flowrates?
4. Is your pressure gauge calibrated?
5. Are you hearing increasing and decreasing(rythmic) sound from the pump?

RE: Operating at extreme left end of Pump Curve

ddkm, something is definitely wrong here. With the pressures and volumes stated, there is nowhere it fits on the pump curve. This may be due to several reasons. Is it the correct curve ? Any reduction on the impeller size ? Any reduction on the number of impellers ? Are the gauges reading correctly ? You may have no option but to physically take dimensional measurements, if the curve is correct. What I also miss here is the speed at which the pump curve stated is measured. If the pump is operated at a lower speed, you expect a curve taking off at a lower pressure of course! (It may have the same profile, but lower pressure range).

You need to check all these.

Again since you said there is a control valve on the discharge, what is the pressure after it ? Also where is the flowmeter located, after the control valve ?

Depending on your answers, a recirculation line may be right, especially since you still need up to 8m3/hr occasionally, but istead of sending it to the suction, it should be sent to a buffer vessel or tank upstream of the pump. This may reduce heating up as it mixes with fresh fluid.

Regards,
Tundelash

RE: Operating at extreme left end of Pump Curve

Some comments as a newcomer to the thread.

1) Pump curve: if the pump is that old, that may no longer be its true curve (maintenance records ok ??). Similar to very old P&IDs seldom representing current configurations (unless a very good doc. tracking/updating is in place which is rare).

2)VSD use
2.1) Why was a pump with that characteristic curve chosen in the first place ?? (is this a "we use it because it was there" situation ?).
If the same design conditions are still valid and a high static head requirement is needed (similar to boiler feed water or oil reservoir water injection applications for instance) with little dynamic flow rate variation requirements (e.g. it runs at a limited number of different flow rates and each runs over relatively long periods of time) then the VSD makes little sense.
A number of pumps in parallel with a suitable Load Management policy (e.g. number of pumps -on- at any given time as per system requirements) would be a more appropriate alternative.
VSDs are usually justified in cases where their particular characteristics as final control elements and/or as energy savings tools are significant. This case appears to be neither.
2.2) VSD vs. PUMP and PUMP MOTOR
When considering a VSD there are issues to be considered with the PUMP. While it is true that Minimum Flow requirement (LOB line) goes down at reduced speeds, there are other issues that may come up at low speeds (e.g. pump shaft defflection, seals temp. and effectiveness etc).
The pump MOTOR also has to be considered for VSD compatibility. If the unit is old, chances are the motor is also old and may need to be retrofitted (re-wound) for VSD use rating (e.g. insulation rating).

If the unit won't be replaced for a more adequate configuration (units in parallel etc.), then Recirculation seems the most appropriate alternative and most comments made so far sound like good recommendations.

Hope this helps (pray tell what you end up doing and how it works).

Regards,
MS

MS
www.3rps.com

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