7 replies to this topic

[suljaga]

 Sketch.jpg   50.85KB   4 downloadsDear, hope so that you are all fine and healthy. We are working on the project in Africa, and We have some issues for which we would love to get your expert opinions.

The condensed water, which condensate at the top of the Glycol regeneration column, is sent to water sump S-01. The condensed water at 50°C is then discharged from sump S-01 by two condensate lift pumps P-01 A/B (intermittent pump operation) at existing water line (1) at TP-01 with normal working pressure of 12 barg. Existing water line (1) pressure 12 barg is constant (back pressure for pumps) because the water is through this existing line send to basin B-01 which is at elevation of cc 110 m. This existing line is full of water all the time and create constant backpressure of 12 barg because of elevation.

Our pumps P-01 A/B are vertical centrifugal type with simple mechanical seals, motor power is 22 kW and the pumps motors are sized for end of curve operation. Two pumps are installed (one in operation, the other one in emergency).  P-01A/B are design for discharge working pressure of 13.0 barg for flowrate of 6 m3/h (BEP). These pumps can be started and stopped locally using the start and stop pushbuttons and also automatic start and stop from DCS is provided. Generally automatic start/stop is main working regime for this pump. The Condensed Water Sump is equipped with a LT-06 level transmitter. This LT-06 transmitter is used to generate low level LAL-06 and high-level LAH-06 alarms which, if activated, generates the following actions: LAH-06 starts the pump P-01A or P-01B; LAL-06 stop the working pump.

During construction phase of the project client added new connection point TP-02 (because the first existing line (1) is temporary out of service) and connect pumps P-01A/B discharge line to additional existing line (on TP-02) which have normal working pressure 0 barg (the line is empty, and it is connected to existing open basin B-02 near our water sump). If we want to send 6 m3/h of water from our sump S-01 through this new line calculated pressure drop is 1,2 bar. Now client want that we provide solution for pumps P-01 A/B automatic start and stop for both cases.

The pumps must have possibility to send water for both cases (through existing line (1) and additional existing line) with automatic start/stop. The first case (with constant backpressure of 12 barg when we send water through existing water line (1)) this system probably can work and pumps can be automatically started by high level at LT-01 as it is design but for the second one (when water is sent through additional existing line) the pumps cannot be started automatically.

During commissioning phase pumps P-01 A/B are functionally tested (4h work each) by manual start with closed discharge valve. Because existing line (1) with backpressure of 12 barg is out of function at that time we must send water to the new line added by client. Pressure at this line is 0 barg and to reach pump working point (6 m3/h and 13 barg) we slowly opening pump discharge gate valve to reach 13 barg pressure at pump discharge. The gate valve opening was only cc 7-8%. We understand that this is not good work, that gate valves need to be in on/off position etc.  but anyway, we must do it even if we had possibility to have bigger problems. The pumps are worked without problems during this 4h testing.

Now we are thinking about possibly solution for this issue. We think that the best solution is to install self-regulating pressure control valve which will control upstream pressure (pump discharge pressure) and keep it at constant value of 13 barg. During automatic start of the pump self-regulating pressure control valve will be in close position and it will start to open after pump is on. Any idea or suggestion is highly appreciated. In attachment you can find the drawing sketch of the system and for any additional questions, clarifications or information we are happy to provide them.

Best regards

 

Edited by suljaga, 24 August 2021 - 05:00 AM.

[breizh]

Hi,

My understanding ,

Pump starts/stops based on LSL or LSH , not LAL or LAL . Don't mix alarms and thresholds . You should have LT with alarms and separate LSL or LSH for automation 

 

BO1 and BO2 should be equipped with LT with alarms and LSL /LSH to actuate the gate valves ( to be automated) .

 

LSL to open the gate ;LSH to close the gate ;LAH to prevent overflow and LAL to avoid to empty the tank.

 

Why do you need constant pressure at the discharge of the pumps ? 

 

Hope this is helping you.

Breizh

[suljaga]

Hy Breizh.

 

You are right for the part: "Pump starts/stops based on LSL or LSH , not LAL or LAL . Don't mixt alarm and thresholds . You should have LT with alarms and separate LSL or LSH for automation ". It is done like you said- there are two separate LTs on the sump S-01 which we did not show in the sketch because we consider that it is not our priority for this topic. B-01 and B-02 are existing basins and they are not in scope of work, they have their own instruments and pumps (I think that it would not help in this case anyway).

We do not need constant pressure at the pump discharge but if we automatically (or manually) start the pump to send water through the new line added by client, with discharge gate valves fully open (gate valve need to be in fully open or closed position), we think that we will have big problems with the pumps because back pressure in this line is 0 barg (the line is empty and elevation of the basin B-02 is el=0m). The flowrate produce by the pump for this case will be out of the pump range and in this situation, we will go out of the pump curve at right side (we will have higher flowrate than 8.5 m3/h which is maximum at pump curve) We are not sure what will happen in this situation and according to our knowledge this is not allowed and there is huge possibility to destroy the pump or motor.

[Pilesar]

I know I did not completely follow the scenario so my suggestion may not make sense: Consider a restriction flow orifice in the new line so that the pump discharge pressure is maintained near the same as when flowing through the full line. That should keep the pump performance similar whatever the line up.

[suljaga]

Hy Pilesar, thanks for answer (you and also Breizh).

 

Our concern regarding the installation of the orifice is that it must be installed on a new line added by client which is cc 70m away from the pumps. Pumps discharge line goes to the pipe brige (6m elevation) and then falling down to TP-02. During the time when the pump is not working, part of the line (from the pump to the orifice) will be emptied due to elevations and at the automatic start of the pump it will take some time for the water to reach the orifices and to establish normal pump disharge pressure (which will give us pump normal working conditions).

[Pilesar]

What would happen if you put an engineered restricting orifice at the pump discharge before the lines split? (I don't know the answer.) It would use more pump energy but might also keep within the pump curve for both situations if sized correctly. 

[suljaga]

Later when they want to use the existing line (1) at TP-01 (see the sketch) and sent water to basin B-01 there will be to much restriction (orifice + elevation)  and our working point will go to the pump curve at left side and pump will produce have very low flowrate.

[breizh]

Hi,

Good to perform an hydraulic calculation with different cases ( B01 ,B02 ,B01 and B02) in service with or without orifice to split the flow .

Notes :

In case of automatic start/stop , you may need a recirculating line to protect the pump(s) in case of both gate valves are closed.

The gate valves should be closed at start up to prevent going on the right side of the curve. 

 

my view. 

Breizh