Closed Cycle Cooling Head Tank Location

[JeffHofacre]

The normal design for closed cycle cooling systems has a head tank on the suction of the closed cycle cooling pumps. For example here would be a typical order of equipment with water flowing through.
1. Head tank tied to suction line of pumps, tank at atmospheric pressure. Water level of tank is 49 ft above pumps.
2. Pumps designed for 150 ft of head
3. Closed cycle heat exchanger
4. Hydrogen coolers ( and other equipment which requires cooling)
5. Back to suction line of pumps

However, I am currently working at a nuclear station that has a different setup. Here is the order of equipment.

1. Pumps designed for 110 ft of head
2. Hydrogen coolers ( and other equipment which requires cooling)
3. Head tank tied into piping. This surge tank is designed for 15 psig. Normal press is 3 - 12 psig. Tank is kept pressurized with nitrogen to a press of 3 psig however the press normally runs 12 psig in the winter. It runs 3 psig in the summer when equipment temperature control valves are wide open. Water level of tank is 80 feet above the pumps.
4. Closed cycle heat exchanger
5. Back to suction line of pumps

Can anyone give a good reason for the second design? With this design the suction pressure varies from 35 psi to 44 psig.

 

[abeltio]

In a closed loop system the elevated tank acts in two ways:
1. allows for expansion of the liquid with temperature
2. acts like a "piston" to pressurize the system, some systems require a minimum pressure to operate (e.g. diesel motor manufacturer's require a min press at the motor inlet) and also to avoid cavitation at the pump suction.
Being a closed loop system, without the elevated tank, the pressure in the piping would have been just the friction losses from the discharge flange to the suction flange of the pump.
The elevated tank pressurizes the whole system.
Where do we locate the elevated tank?
It does not really matter, being a closed system we can "cut" the pipe anywhere and each side would be the "discharge" piping or the "suction" piping for the pump
My preference is to locate the the elevated tank tie in as near to the pump suction flange as possible...why? if there is a malfunction in the system (e.g. blocked heat exchanger, valve etc) the elevated tank will continue to supply positive head to the pump suction.
If the tank tie-in can get incommunicated with the pump suction there is a possibility of starving the pump and go into cavitation.
In any case the system should detect a low flow condition and trip the pump to avoid damage due to overheating.
HTH
saludos.
a.