8 replies to this topic

I am designing the P & ID for a cooling tower.

(1) Are there any resources/guidelines available that might be helpful in designing the cooling tower system (i.e. cooling tower, the pumps and associated piping)?

(2) There are three cooling water pumps, two in operation - one in standby. Under what condition would you see a drop in discharge pressure? If one of the operative pump trips, what effect will it have on the discharge pressure?

The reason for my question is, should I install a low flow switch or a low pressure switch on my discharge line? The activation of this switch should start the standby pump

Thanks

Meghal

[sgkim]

To develop P&ID for cooling tower system the following should be considered in general.

(1) Cooling tower
-configuration, type, number of cells, type of water distributor
-type of fans and temperature (speed or stroke) control methods,
-balancing(vibration) monitor and/or interlocks,
(2) Chemical dosing system
-chmicals to be used
-injection point and chemicals flow control
-water quality(pH, conductance, dissolved solids, etc) monitoring
-blow-down flow control
-sludge removal method
(3) Cooling water pumps
-type and number of pumps,
-supply pressure monitoring
-power, vibration, motor temperature monitoring
-operation mode
-mechanical seal schedule
(4) Circuit configuration
-supply & return loop
-temperature monitoring
-header pressure control and/or minimum flow bypass loop
-hot bypass line and flow control method
-blow-down point and destination (to wate water treatment)
-make-up flow control to maintain the level of water basin

If one of two pumps trips, then the flow rate will drop to about 1/2, and the discharge pressure
will drop to about 1/4. (Q ≒ f( (pressue loss)^2 ). You can employ any of pressure switch(es), motor trip signal(s), or flow switch to start one of stand-by pumps. Pressure switch would be preferred to maintain constant supply header pressure. Header bypass control valve may limit high supply pressure. You have to specify the start-up priority if several pumps are under stand-by for adative load control.

Stefano/051107

[fallah]

Dear All
As per attached 3D dwg. we have a wet cooling tower with 3 cells which fed by 5 parallel (4 working+1 standby) recirculation pumps (PU.ME 45 A/B/C/D/E).As we see in that dwg. a common header connects the discharge lines of pumps to inlet lines of cooling tower.
Temp. of water fed to cooling tower = 60 C
Elevation difference between discharge nozzles of pumps and inlet nozzles of cooling tower cells = 6.2 m
Q(total)=3940 cubic meter per hour
Q(each pump)=985 cubic meter per hour
Head of each pump=2 barg
Size of each pump discharge line=DN 400
Size of common header=DN 750
Size of each line between header and each cell of cooling tower=DN 450
Based on a.m. data and information available in attached dwg. ,can we expect that the flowrates of water fed to 3 cells are equal (each line =1313 cubic meter per hour)?
Warm Regards

Attached Files

•  Cooling_tower_circulation_pumps.pdf   125.04KB   456 downloads

[marthin_was]

I believe what you need on your system, is to maintain cooling water pressure on the header line, not the low flow, because its to difficult to determine low flow set point, related on your process. I think, if the pressure less then 20% from your normal operating pressure, is a good starting point as set point to run standby cooling water pump.

When you want to design cooling tower, usually it is a spesific job, that can be done by vendor. BUt for rough calculation, I already search on GOOGLE, and many sources you can refer to...


thanks,
Marthin

To develop P&ID for cooling tower system the following should be considered in general.

(1) Cooling tower
-configuration, type, number of cells, type of water distributor
-type of fans and temperature (speed or stroke) control methods,
-balancing(vibration) monitor and/or interlocks,
(2) Chemical dosing system
-chmicals to be used
-injection point and chemicals flow control
-water quality(pH, conductance, dissolved solids, etc) monitoring
-blow-down flow control
-sludge removal method
(3) Cooling water pumps
-type and number of pumps,
-supply pressure monitoring
-power, vibration, motor temperature monitoring
-operation mode
-mechanical seal schedule
(4) Circuit configuration
-supply & return loop
-temperature monitoring
-header pressure control and/or minimum flow bypass loop
-hot bypass line and flow control method
-blow-down point and destination (to wate water treatment)
-make-up flow control to maintain the level of water basin

If one of two pumps trips, then the flow rate will drop to about 1/2, and the discharge pressure
will drop to about 1/4. (Q ≒ f( (pressue loss)^2 ). You can employ any of pressure switch(es), motor trip signal(s), or flow switch to start one of stand-by pumps. Pressure switch would be preferred to maintain constant supply header pressure. Header bypass control valve may limit high supply pressure. You have to specify the start-up priority if several pumps are under stand-by for adative load control.

Stefano/051107

[fallah]

Dear Marthin

We have specified the cooling tower for vendor and the vendor designed the cooling tower.As usual ,the battery limit for vendor was inlet nozzles of cooling tower cells(inlet of water distribution system) and design of inlet header and pumping system (along with its piping) were not in vendor scope.
What now we investigate is, whether considering common header between pumps and cooling tower and also unsymmetrical situation of header inlets with respect to header outlets affect the equal distribution of water into 3 cells of cooling tower?

Regards

[AA Mishra]

You never knew Engineering Contractor to get P&ID, resources/guidelines.

You never saw pump discharge NRV malfunction, discharge header leaks, pump supply reservoir empty, shortcircuiting through standby pump in line.

You never saw flow requirement on one of the operative pump tripping.

You never knew these kind of pumps start - stop is field opeation.

You never knew to confirm installation of low flow switch and low pressure switch as indicative.

You never knew start of standby pump on activation of switch is unsafe.

You can neither confirm nor you can answer.

You can repeat the statments of the people.

You can help.

Good.

Very Good.

Regards

[fallah]

You are advised, poetically speaking has never helped to engineering analysis of the issues.

[Travesh]

Hi Meghal,

I am busy with similar design work for a large cooling tower project. As a starting point for the PID, I suggest you draw a Process flow diagram (PFD). Thereafter, PID can be drawn that is consistent with the Client's requirements including:
1. The amount of cooling tower cells and capacity (heat duty)
2. The amount of pumps and capacity (head and flowrate). You generally have open circuit pumps and closed circuit pumps that are linked by plate heat exchangers. Please confirm for which type of industry this is (eg power plant etc).The flowrates and Client specified pressure drop will set the line size.
3. How many consumers of cooling water are there?These consumers will user cooling water from the closed circuit header at required flowrates for cooling.
4. The specified cycles of concentration (COC). This will set the flowrate of the blowdown and make-up and therefore piping size.

With respect to one pump tripping, you need to do a careful analysis of this effect on the pressures in the pipelines (called "hydraulic transients". The pressure will not necessarily decrease but could Increase momentarily depending on your system. If the pressure does increase then make sure that this pressure does not exceed the design pressure of the piping. A good program to use to simulate hydraulic transients is AFT Impulse. This considers the deceleration of the fluid and this effect on the transients.

Good luck
I am designing the P & ID for a cooling tower.

(1) Are there any resources/guidelines available that might be helpful in designing the cooling tower system (i.e. cooling tower, the pumps and associated piping)?

(2) There are three cooling water pumps, two in operation - one in standby. Under what condition would you see a drop in discharge pressure? If one of the operative pump trips, what effect will it have on the discharge pressure?

The reason for my question is, should I install a low flow switch or a low pressure switch on my discharge line? The activation of this switch should start the standby pump

Thanks

Meghal

[katmar]

I have deleted the comment I made yesterday because I had not noticed that Travesh had dug up a 3 year old thread. Please guys, let's not dig up these ancient threads anymore.

Edited by katmar, 19 October 2010 - 01:57 AM.