7 replies to this topic

[paradox7]

I am having a problem with priming a mag drive seal less pump. I have attached a basic drawing of the system ( http://dl.dropbox.co...off-loading.bmp ). 15% Sodium hypochlorite is discharged from a tanker through PTFE lined pipe to the pump, which pumps the liquid to the vented storage tank. There is a check valve on the line with a bypass. During the attempts of priming the pump the spring loaded by-pass valve is opened in an attempt to release any air lock. However the pump trips out (believed to be caused by dry running). There is not much head differential between the tanker and the inlet line.

There is scope to modify the system so that the pump will be primed correctly. I am looking for ideas to what could be modified to rectify this issue.

I am a new process engineer with less 1 year experience in industry. Any help would be appreciated.

Edited by paradox7, 12 February 2013 - 06:35 AM.

[kkala]

15% aq NaOCl has quite small vapor pressure (*), no NPSH problem is anticipated on the condition that the liquid arrives at magnetic driven pump suction inlet. Not to say that insufficient NPSH may not trip the pump immediately. Look into existing controls tripping the pump (assumed centrifugal), trying to guess which of them is activated to stop pump motor. Probably first actions could be:
1. Check for excessive amperage at pump startup, which could trip the motor. If so, start the pump with discharge valve closed, then open it gradually in time proved enough (say 1 min). Bypass on check valve has not been seen here, at any case it does not affect mentioned procedure.
2.  Check whether pump inlet is vertically lower than truck container bottom (plus some safety margin). Suction flow could be realized through siphon even if pump inlet is higher (at any case lower than truck container high liquid level); but siphon can be unstable, also it might not be restored once interrupted.  You may need to "elevate" the truck a bit or "lower" the pump.
Best for PTFE suction pipe to have almost steady slope from truck container nozzle (assumed at bottom) to pump inlet (not ups / downs). Check also that container compartment being emptied is under atmospheric pressure.
3. Cause of the trip (excessive amperage, no discharge pressure) may be seen on DCS control panel, which helps diagnosis. In case of no diagnosis, check also NPSHa.
4.The option of replacing existing centrifugal pump (flow rate?) by another being self priming has not been looked into.
Comments / additions on the above are welcomed. What can be the "safety margin" referred in para 2? Do truck compartments have vortex brakers?

(*) see <http://www.atekwater...sodium_hypo.pdf> as an example

Edited by kkala, 12 February 2013 - 11:40 AM.

[paradox7]

Thank you for your detailed reply. Yes I dont believe it is a NPSH problem either. I believe the pump (centrifugal) trips due to high amps. 1.Not sure why you would start the pump with the discharge valve closed? 2. There is about 2m head difference between the top of the liquid in the tanker and the pipe inlet. But from the tanker outlet to pipe inlet maybe only 0.5m 4. I dont believe there are vortex brakers in the tanker but I will look into this. Getting a self-priming pump may be an option but want to try and solve by cheaper alternative if possible. The flowrate is 25m3/hr

[kkala]

1. Normal way to start a centrifugal pump (except axial flow pumps, not considered as fitting case here) is with discharge pump closed, then gradually opened, <http://www.cheresour...p-backpressure/>.  In this way you can avoid "thermal" trip of the motor (due to excessive load).   Besides measurement of absorbed motor power could give useful indications, even when pump is managed to be put in operation.
2. Vertical distance of 0.5 m (minimum) between container bottom and pump suction can be adequate.  Similar small truck unloading  configuration is roughly recalled at plant (~1979), but concerning diesel.  Nevertheless  priming problem appears at start up, when truck container  must be rather full.
Hopefully the problem will be soon solved.

Edited by kkala, 13 February 2013 - 01:55 PM.

[thorium90]

Normally when a pump or compressor is started, it will initially draw a higher current to provide the torque needed to turn the shaft. After starting, the pump will then draw power proportional to the volume pumped. By having the discharge valve fully open at start,  the initial current draw will add to the full flow from the fully open discharge valve will almost certainly exceed the amperage allowed and trip the pump. Its not advisable to leave the pump discharge fully closed either as the gas lock will not get removed. Best practice is to have the discharge valve crack open during startup. Once the downstream pressure is more stable which is an indication the airlock is cleared, then proceed to start opening the discharge valve more.

 

If the pump is completely gas locked at start, (no liquid inside), it will also draw excessive power.

Edited by thorium90, 13 February 2013 - 02:42 PM.

[kkala]

The procedure of starting a centrifugal pump against closed (nearest) discharge valve is considered as standard in a lot of applications.  See posts 12 to 21 in mentioned link of post no 4 (by kkala). Before start up, all pump + suction and discharge line to the valve should be full of liquid, which seems possible in the discussed case, and progressive opening of the valve will not delay, since it has to be fully open in (say) 1 min.
On the other hand starting with crank open valve does not seem to have a significant difference. See < http://www.enggcyclo...t-up-procedure/>, <http://in.answers.ya...02093249AARuqIU>. The closed valve of first case will become a crank open valve a few seconds later.

In  case of a pump running dry (no liquid in it) differential head will be zero and mechanical friction is expected to increase. What about the motor power? Any note from literature or even observation?

[thorium90]

There is a difference here because this pump here is connected to a tanker and is disconnected after use every time which may allow air to enter whereas the ballast water pump always has water at its suction. If there is air in the suction line and if the discharge valve is fully closed the bubbles will collect at the impeller area. For the ballast water pump, it is ok to start with discharge fully closed since there is no risk of air in the suction.

 

From your second link (the one from yahoo answers), the last paragraph of the best answer says:

"If you operating the pump for the first time , then discharge line needs to be primed by keeping the vent valve or end valve in open condition."

 

Due to the nature of the NaOCl, I assume there is no vent valve to remove trapped air, so the only way is to have the discharge valve slightly open at start.

 

If there is no liquid at all, it will draw very little power but it will become very hot very fast as there is nothing to cool the seals. But as this is a seal less pump, it might be different..

[kkala]

My understanding from second link, thorium90, is that you start the pump motor with the discharge valve closed, then you gradually open the valve to "prime" (meaning here "fill") the discharge line. Before that you should fill (=prime) the pump with liquid. Air in both cases can escape through tank breathers (or vents to safe location).

However crank open valve is also OK, since it is not expected to trip the motor. It may also help to remove entrapped air (downstream discharge valve) earlier, if some of it has remained. 

The latter is also supported by the two links of post no 6. First of them says "fully close or partially open the discharge valve, depending on system conditions", while the other "In smaller capacity pumps, you can start the pump in circulation..." (not met by me in practice).
Reported start-delta electrical connection affects dips during first milliseconds of startup. Trip of such kind (often forcing us to change connection) appeared only in big plant motors for crusher, rotary drier, etc. Trip due to motor overcharge (thermal trip) had a relatively longer delay. Plant fans were sensible to thermal trips (transfering cold air at startup), pumps not so. Of course this depends on the installed motor margins.

Seen pumps frequently running with entrapped air had heavy maintenance, but were not destroyed immediately, look at <http://www.cheresour...dry-run-of-pump>, mainly post no 5 (by kkala).

Edited by kkala, 18 February 2013 - 08:57 AM.