Closed Loop Piping-Vacuum Issue

[DurgaprasadM]

The lube oil present in the shaft bearing sump has to be cooled externally using a heat exchanger. A gear pump is being used to circulate the lube oil from sump to the heat exchanger. This is a closed system. During the initial startup, the vent valve will be used to purge all the air in the circuit. The moment gear pump is switched off, it is expected that there will be vacuum in the system above the lube oil level (Magenta line). During the next start up of gear pump, is there any problem with this vacuum? If yes, what is the solution.

Note: I cannot able to provide a header tank as there is restriction of lube oil level in the sump of shaft bearing.

 

[Doug Hunter]

To answer your question directly, I don't think that the vacuum would cause a problem in the gear pump start up, unless it caused a start-up torque that exceeded your motor capability (unlikely). However, I'd also like to elaborate on your premise: unless your gear pump has some unusual sealing mechanisms, the suction vacuum will decay over time after shutdown (probably over only a few seconds) and the system pressures will equalize.

 

[LittleInch]

There might be low pressure, but unless your oil is close to its vapour pressure or the height of the line to the top of the HX is more than 8 or 10m, this should not result in any actual "vacuum" only a lower pressure due to the static head.

I have to say this does not look like a classic "closed loop system" to me with the variable being what the flow is in the HX and this volume in the bearing. There needs to be some allowance for fill, drain and pressure relief / regulation.

Does that help?

 

[GD2]

The lube oil present in the shaft bearing sump has to be cooled externally using a heat exchanger. A gear pump is being used to circulate the lube oil from sump to the heat exchanger. This is a closed system. During the initial startup, the vent valve will be used to purge all the air in the circuit. The moment gear pump is switched off, it is expected that there will be vacuum in the system above the lube oil level (Magenta line). During the next start up of gear pump, is there any problem with this vacuum? If yes, what is the solution.

Note: I cannot able to provide a header tank as there is restriction of lube oil level in the sump of shaft bearing.

Is there a reason the pump inlet has to be from the top of the sump? If you flip, the lines (pump draws from the bottom and cooled oil delivered at the top), the pump will not only avoid any vacuum condition but will also have reduced work done (backpressure will be reduced).

 

[DurgaprasadM]

Doug Hunter - Good day! Thanks for reply. I am not worried about the pump performance. If there is entrapment of air into the pipeline by any means after the gear pump is switched off, I have to purge this air every time after start-up of gear pump?​

GD2-Good day! Thanks for reply. The lube oil sump suction and discharge points are almost at same elevation.I really want to know do I need to purge the vent valve every time during the startup of the pump.​

LittleInch -Good day! Thanks for reply and for the insight provided that there is no vacuum but the vapor pressure of the lube oil present in the system above the lube oil level.​

 

[Doug Hunter]

Good morning. You'll have to consider the effect of air on the performance of the heat exchanger and the bearing, but in my experience the pump and the entire system will tolerate the presence of some air. High pump speed and high discharge pressure can create an increased risk of cavitation damage, and the presence of air can sometimes make this worse (or sometimes make it better). Can you share the pump displacement and speed, and the typical discharge pressure?

 

[georgeverghese]

Suspect you dont have a check valve immediately downstream of the gear pump discharge, which is why the pump discharge riser sees reverse flow and fills up with air on pump stop.
See if you can install a check valve on pump discharge.

 

[Snickster]

Considering you have the system up and running with piping full of oil with the sump partially filled with oil under steady state conditions I would think at this time there will be atmospheric to a partial vacuum in the sump, depending on how much the sump is sealed from the atmosphere, and positive pressure on the pump discharge. When the pump is shut down there will still be the same pressure as was during steady state in the sump. The oil in the heat exchanger and pump discharge will want to drain back to the sump but this will cause a vacuum to develop in the system high point stopping the fluid from backflowing into the sump. However as LI mentioned if the maximum height of the piping is high enough the vacuum created will not be enough to stop the fluid from backflowing into the sump. In this case some of the fluid will backflow and will be replaced by oil vapor at its vapor pressure.

In any case whether the fluid backflows or not when you start up the pump again, I believe you will reach steady state pumping condition again without any issues. Any vapor spaces of oil vapor that may form in the piping will collapse when the pump is restarted and piping will flow full, and the sump oil level will return to normal.

 

[Doug Hunter]

Doug Hunter - Good day! Thanks for reply. I am not worried about the pump performance. If there is entrapment of air into the pipeline by any means after the gear pump is switched off, I have to purge this air every time after start-up of gear pump?​

GD2-Good day! Thanks for reply. The lube oil sump suction and discharge points are almost at same elevation.I really want to know do I need to purge the vent valve every time during the startup of the pump.​

LittleInch -Good day! Thanks for reply and for the insight provided that there is no vacuum but the vapor pressure of the lube oil present in the system above the lube oil level.​

Good morning. The answers to your latest questions about air depends somewhat on the details and air robustness of the heat exchanger, but generally I don't think that you need to purge every time. What I can tell you with certainty is that as long as the gear pump suction line at the sump remains completely immersed in oil (or an oil/air solution that is 75%+ oil), then a properly-working gear pump will push oil through the heat exchanger, along with any air in the lines. The sump fill must be adequate to cover the pump suction even with the heat exchanger full of oil. Upon shutdown, assuming that the total system height is within ~1 meter or so, the pressure throughout the system will equalize within seconds, and all the oil will drain back to the sump to the degree allowed by the geometry of the oil passages (excluding any U traps), and any air in the system will expand or compress as needed to fill the space, and you are very unlikely to have any residual oil vapor after a few seconds. There are 3 modes for the air: (1) free air, (2) air in solution in small bubbles, and (3) air absorbed/dissolved between the oil molecules (see Henry's Law). The proportion of each at any time or location in the system will depend on temperature and pressure. Because of this you are unlikely to be able to ever remove all of the air from the system, but only modes 1 & 2.

 

[GD2]

OP,
Looks like the schematic is not complete. I am wondering where is the purge valve and how it will be controlled/commanded? As Doug has pointed out there will be air in the system from various sources ( I would add leak to it) and you must have a design in place to automatically test the system for air, pull out the air or even separate the oil+air before throwing out air.
You don't want to see any air in the system. Correct? If that's the case and if it needs an automatic control, you will need a computer controlled system.