9 replies to this topic

[processji]

We have a waste water sump with centrifugal pump taking suction from the sump and transfering waste water to a treatment unit. Pump max capacity is 250 m3/hr.Max temperature is 50 Deg C. We have a recircualtion line with a control valve on it. We have a globe valve to block in flow to waste water treatment plant. In such event we would re-circulate water from sump to ensure mixing. In normal case the water from sump will go to treatment plant. The main line is 12" and recirc line is 6".

My question is:

1.0) If i consider recirc valve to be linear charecter valve size need is 8". If i consider equal percent valve size turns out 6". What charecterstic should be chosen?
2.0) Will the valve a flow control valve.
3.0) Is there a guideline for % recirc.

Thanks
Regards
K Suresh

[Luigi]

The main reason for recirculation in pumps is to assure that the pump works at least at the minimum flow rate. If not, you could have overheating (that can even get to the point of cavitation) and premature failure of the mechanical seals or bearings). Under this scenario, the manufacturer will tell you what the minimum flow is. It can vary greatly. I've seen from less than 10% to more than 40%.

However, in your case the recirculation fill another purpose, assure an adequate mixing. Under that situation, you should find is there is a minimum requirement to satisfy this purpose.

I couldn't understand if the control valve is existent or you are planning in installing it. How is it controlled?

Side note: Is there a reason why you have a globe valve at the discharge of the pump? Unless you really need to do manual flow control, you might be wasting energy if compared to installing a gate, butterfly or ball valve.

[JoeWong]

suresh786,
My 2-cents worth comments...

1.0) If i consider recirc valve to be linear charecter valve size need is 8". If i consider equal percent valve size turns out 6". What charecterstic should be chosen?

The purpose of minimum recirculation line is for centrifugal pump protection from cavitation. Luigi has clearly explained this.

This is a liquid service. I would think a linear characteristic CV may be sufficient. I am not INST specialist. You may confirm with you INST engineer.

Why not an RO on recirculation instead of CV ?Required Flow is 250 m3/h, assued dP (not specified) across pump is 3 bar, with shaft efficiency of 60%, shaft power will be about 35 kW. A recirculation control valve is a good option compare to permanent RO (per my rules of thumb).

However, you have additional requirement : Mixing via recirculation
Please ckeck your requirement on recirculation on mixing. If the recirculation flow for mixing purpose is more than minimum recirculation of pump, you may consider to have a permanent RO.

2.0) Will the valve a flow control valve.

Yes. Typical design is flow meter on pump discharge, feeding signal to recirculation CV. Once flow meter detected pump discharge flow is lower than set point (minimum flow protection flow or required flow for mixing purpose, whichever is higher), it will start to request recirculation CV to open in order to increase pump flow to its setpoint.

3.0) Is there a guideline for % recirc.

If your pump is existing pump : I am almost certain that pump datasheet will indicate the minimum flow requirement.

If this new facilities : Luigi has indicated 10-40%. A good start. General i use 30% as my first guest.


12" line with 250 m3/h will give about 1 m/s. Looks small. 10" line may give you 1.4 m/s. Still looks OK. What's your criteria for line sizing ?




JoeWong

[fallah]

How the minimum flow valves works, once the pump discharge flow is decrease to set-point, the valve will fully open and when the pump discharge flow is above the set point the valve is fully close? If this is the case, then better just use the on-off valve. Please clarify.

Once the flow in pump discharge line decreases from a prespecified value the min flow CV begins to open such that to compensate flow shortage.When the flow in discharge line goes up above set point the CV would be fully closed.By this control valve you compensate flow shortage in any case but using on-off valve you wouldn't perform any control on flow in dicharge line and in the case of flow shortage many sequential on and off may be occured.

[fallah]

Fallah,

Assuming that the minimum flow requirement of the pump is 20 m3/hr and pump normal flow is 60 m3/hr.Is that means at pump discharge flow of 20 m3/hr then minimum flow recycle valve should be 80% open to meets the minimum flow requirement (max. control range)? So at what flowrate the control valve should start to open?

In your mentioned case,the set point for opening of min flow CV (these type of valves are normally closed) would be,let say,22 m3/hr (2 m3/hr as a safety margin) and this CV try to maintain this flow in main discharge line.

If the flow decreases from 22 m3/hr at a point,let say,21 m3/hr there is LAL(level alarm low) on relevant FIC announced the operator for proper action.

There is another layer of protection as LALL (with a value of 20 m3/hr or a little bit) on dedicated FT in discharge line trigger an interlock and would trip the relevant pump.

Hope above heps you out

Edited by fallah, 31 May 2010 - 01:21 AM.

[Robert Montoya]

The value of this minimum flow given by the manufacturer, is maximum 40%. As a preliminary solution assume that all pump of the power in conditions of blockade is converted in heat, then provide a flow for a 15 ° F increase in temperature of the flow.

[chetanghotekar@gmail.com]

Hi Suresh,

As far as Pump Recirculation application is concerned, we should note that the pressure drop across this valve is more or less same for all flow conditions(fixed pressure drop, because the outlet is discharged to a sump or an open air tank). For Fixed pressure drop applications, Linear characteristic is recommended.

Second thing to find out, based on application, is, whether the valve will be required to modulate or just ON-OFF, In your application, it is mentioned about "mixing", so you need a modulating valve. Valves on these applications are normally closed and need to have tight shut off (MSS-SP-61)in order to avoid "wire drawing effect" (fluid creating a leak path on the seat) & also to avoid any unnecessary fluid getting recirculated due to leakage (causing waste of pump energy).

More severe the pressure drop (typically above 25 - 30 bar), there are always good chances that cavitation or flashing may occur (generally outlet pressure is very low for recirculation valves). Typically multipath multi satge valves are suitable for such applications.

[kkala]

Some further thoughts (based on local work "habits") of probably general interest:
1. We specify flow control valves for pump minimum flow lines without tight shut off, due to (assumed) high cost of the latter. Yet I see that tight shut off may be recommended in some cases, e.g. high ΔP (slurries??).
2. Suppose that min flow requirement is 30% for the 250 m3/h, then ideally the min flow valve shall have 0 m3/h at conditions of 75 m3/h pump flow and 75 m3/h at conditions of 0 m3/h pump flow (i.e. discharge valve closed). For e.g. 40 m3/h pump flow downstream of min flow branch, required flow through the valve is 35 m3/h.
Above have some margins in practice to ensure pump min flow in all cases. Mechanical min flow devices, e.g. yarway type or equivalent, are considered safer (cases have been detected with such valves always opened due to malfunction).
Note: Process leaves these margins to Instrument Department, noting the valve function.
4. RO (orifice)on the min flow branch will have return flow continuously, contrary to min flow control valve. This is usually an advantage of the latter. As a standard practice, we do not use RO for min flow.
5. Suppose you have a PRV at pump discharge. PRV flow required and set pressure will not take min flow into account (min flow valve not considered reliable enough).

Edited by kkala, 26 June 2010 - 05:31 AM.