Initial Filling of Empty Space Downstream of Seawater Pumps 3

[jmas]

Would someone please tell me why we always first-fill empty spaces of desalination units (or once-through cooling systems) downstream of main seawater pumps? In my thought, it should relate to compressibility feature of air inside of empty space and that emptying that air with high speed make perhaps a lot of noises or damage something. Any explanation?

 

[BigInch]

It might be so you don't go to runout with the mains.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[CH5OH]

its to dearate your system. if coolers are allowed to entrap air, it not only reduces their cooling capacity, it also speeds up fouling.

 

[JJPellin]

Three reasons occur to me. The first one was already pointed out by BigInch. It would be a bad idea to run the main pumps at excessively high flow while you pack the line. Running at the "end of curve" can result in high thrust loads (depending on the pump configuration) and cavitation.

The second reason is the same reason that we don't pressure test new piping with air. We pressure test with water (hydro-test). High pressure gas is much more dangerous than high pressure liquid because of the compressibility of the gas. If you had a leak or a pipe rupture, you don't want to have that line full of pressurized gas.

The third reason is water hammer. When the liquid hits the end of the line and the line becomes liquid packed, there can be a pressure wave as that liquid has to decelerate. That pressure wave can blow the top off a valve, crack the line, rip pipe supports loose, etc. You need to be careful of the fluid velocity at the point where the line becomes full.

A vacant line should be packed in a controlled manner that will not do harm to the line or the pump.


Johnny Pellin

 

[CH5OH]

good practice is to start a pump with a closed discharge valve to prevent the banging or high amps pull

 

[jistre]

Oddly enough, I'm learning about RO systems for a possible upcoming project, and just read startup procedures of RO desalination plants.

The system is filled at 30-60 psi with the high pressure pump turned off and the product and waste valves open to slowly flush out any air, as it can damage the membranes if trapped. Even when the HP pumps are started, the valves between the pressure vessels and the pump are slowly opened to prevent large pressure differentials that can damage the membranes. Also, the pressures need to be carefully brought up to allow the permeate and concentrate valving to be set properly to design specs so the system doesn't recover too much water from the concentrate, as the salts left behind in the concentrate could possibly become concentrated enough to quickly scale out and damage the system or reduce output.

Everything you ever wanted to know about RO systems...

http://www.dow.com/liquidseps/service/lm_design.htm

 

[jmas]

Thanks a lot for responses. In most of answers it is assumed that end of line is closed but it is not so. In case of combined cycle power plant (once-through cooling), seawater pumps discharge to a conduit which guide water to the condenser and then to the outfall (so there is not dead end there). Also, in MED desalination (which is not RO), the air is not blocked, but in all of them air entraped at the highest point will be vented by a mechanism (priming system or air valve). I should be grateful if we have a more physical answer.

 

[JJPellin]

It is not necessary to have a dead end in order to have water hammer. Any pressure drop will do: A pinched control valve, a heat exchanger, a diameter change, an orifice plate for a flow meter, another pump in series further down the line. And the concern about running the pump at excessively high flow while the system fills is not dependant on a blocked end. The back pressure on the pump will not be established at normal levels until the system becomes liquid packed. Before that, the pump flow may be too high, resulting in cavitation or thrust bearing failure.

Johnny Pellin

 

[jmas]

Thank you Johnny. it seems i'm going to have a better understanding. But til now i thought that cavitation is "long-term" problem, I mean for, say, 10 min. of rapid filling what problem may Cavitation brings up? or in this short duration do we really damage the bearings (all of them i think is refered to main pumps)? Could you please describe in this case the physics behind water hammer? In our academic study, water hammer in pipeline was due to elongation of pipe and compression of water.

 

[BigInch]

Again I think its just for runout protection.

While waterhammer can occur without blocked in segments, its effects would be corresponding smaller then when stopped by a rapidly closing valve and, if releasing air through a properly sized vent valve such that little air pressure is built up, it takes us back to avoiding runout conditions. But, it could also be that the vent valve is not sized for venting air at faster pump runout flowrates, in which case "2-phase waterhammer" might also be avoided. Air compressed by excess water momentum to such an extent that the water flow is temporarily reversed until air pressure reduces. Hey. Stranger things have happened. But the root cause would be high runout flowrates, so I'd revert to that reason.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[JJPellin]

Perhaps an example may help: Filling a long line up to a receiver vessel that is a long distance away and at a higher elevation than the pump. At the other end of the line is a flow meter that consists of a meter reading the pressure drop across an orifice plate. After that, there is a control valve that is attempting to hold constant flow in the line. The vessel is vented to flare so that no gas pressure builds up ahead of the liquid. We start with a vacant system and start up a large circulating pump. With no back pressure, the pump runs out to the end of the curve which may be twice the normal flow rate. The flow meter at the other end does not register this flow because only gas is passing through the orifice plate. An equivalent volume of gas does not produce much pressure drop as compared to water. Since the meter is sensing low flow, the control valve goes wide open attempting to reach set-point. The velocity of the water coming up the pipe is twice the normal velocity since the pump is running out to the end of the curve. When the water reaches the orifice plate, there is a sudden increase in pressure drop across the orifice plate. But, the column of water coming up the pipe cannot slow down instantly. In fact, the pressure wave coming back from the orifice plate can only travel at the speed of sound. So, the column of water keeps moving forward at the original high velocity until the pressure pulse gets back to the pump to push it back up on the curve. Since water is basically incompressible, an incredibly high pressure may be achieved before the pump flow drops off. The longer the line, the higher this pressure may build since it will take even longer for the pressure wave to travel back to the pump. In addition to the immediate pressure wave, when the flow meter senses the high flow, the control valve will close to try and achieve set-point. The rapidly closing valve could produce another pressure wave that would also have to travel back to the pump before the pump flow could be reduced. The resulting pressure spikes could be many times greater than the shut-off head capability of the pump. It could blow the line apart. This same event could happen if there was a heat exchanger, filter element, pinched valve, or reducer in the line.

You have never described the main pumps in your system. The problem with operation at the end of curve depends on the pump. Another example may help. Our largest water pump is a 10 stage barrel pump. All 10 impellers are in series, oriented in one direction. In order to counter the extremely high thrust that would result from these 10 impellers, there is a balance piston on the discharge end. The balance piston uses a pressure differential between suction pressure on one side and discharge pressure on the other side to counter the thrust of all 10 impellers. This particular pump normally runs with about 3600 psi discharge pressure. If we were to operate the pump with a wide open discharge line and no back pressure, there would be no discharge pressure against the high pressure side of the balance piston. Operating in this mode, the individual impellers would also be producing less thrust. But, there could still be many thousands of pounds of thrust produced by the impellers that the balance piston could not counter. Running this way for even 10 seconds could result in a thrust bearing failure.

Cavitation could be viewed as a long term reliability issue. But, water is the most destructive fluid for cavitation. Running in hard cavitation for 10 seconds at each start-up could result in cumulative damage to the first stage impeller. Depending on how often the process is started, this cumulative damage could be very significant. Also, while running in hard cavitation, the shaft deflection would be greater. I would expect an increase in bearing and mechanical seal failures by running in this mode.


Johnny Pellin