13 replies to this topic

[vicky123.]

Dear all

I have a problem with unloading of a rail car wit our existing vaccum system. We have a facility of unloading railcar filled by caustic soda lye through vaccum system.

acctually from some days I faced some truble when unloading, the unloading pumps are not pumping properly. for undurstang our existing system use the attached file.

 caustic unloading system 1....bmp   1.52MB   57 downloads

Here:
1- liquid filled rail car
2-first stage reciver tank at which unloaded material by the attached vaccum pump is collected
3-vaccum pump which can create maximum of 650 mmHg vaccum in ideal stage and this is the only driving force in our system which pull out the material from railcar to the reciver tank.
4-two numbers of unloading pumps which are used to take suction from the reciver tank and delevered the material to the final storage tank
5-Non retunable or non reversivel valve
6-storage tank

As i say, at the time of operation the vaccum is around 600 to 650 mmHg. As per my doubt due to the vaccum inside the receiver, NPSH may fall very low so dose the unloading pups stopped working.

please merccey me for my bad english and plese tell me if u have any suggesion.

thank u

[breizh]

Vicky ,
You may find some interest reading this pamphlet (Dow chemicals).

Hope it can help you ,

Breizh

[vicky123.]

dear sir
As i mention in my post our system running through only vaccum there are nither any pressurised air to facilited the unloading nor we can use unloading from bottom of the rail car as orr unloading infrastructure dose not support it, and the attached file described mainly these two system, so please can u suggest me any modification in our existing system to improve it.

always your thank full
vicky

[breizh]

After transferring to the receiving tank you need to break the vacuum and then operate the pump. Be careful with this dangerous material , full set of PPE is required .


Breizh

[Ajay S. Satpute]

Hi Vicky,

1. Does receiver tank have any pressure gage to confirm it has got no vacuum when you attempt to unload the material?
2. Are you sure there is no additional (unexpected) pressure drop in the suction or discharge line due to line blockage, inadvertant (partial or full) closure of manual or automatic valves etc.
3. Does the pump discharge line have pressure or flow meters to ensure pump follows the pump curve?

Regards.


Ajay

[vicky123.]

dear ajay ji
The reciver tank under vaccun at the time of unloiading but the total liquid head in the reciver is greater then the vaccum so the unloading pump get the some npsh. In past we unloaded materials easyly even if the reciver vaccuume gauge show value around 600 mmHg, but now a days due to some reason after completed 3 to 4 rail car the vassel got filled and the unloading process stoped. Again if we epty out the reciver tank by drained it unloading process started normally and again after some time found the vassel full.
I can say confermly there is no blockage problem and at discharge of the unloading pump there is pressure gauge which shows 5 kg/cm2 pressure when unloading run and 0 kg/cm2 when unloading process interupted.

[Ajay S. Satpute]

Dear Vicky,

This is getting interesting.

1. Can you post P&ID of this system if available?
2. Is there a stand by or spare pump available which can be used to confirm the problem is with the pump or not?
3. I suggest you carry out the pump sizing calculation including suction and discharge line pressure drop calculation. This will provide you more insight due to availability of calculated quantitative data. Try work with pump curve (if available) and compare the (head and Q) values with the calculated ones.
4. A few years back I was employed in tar distillation plant, where coal tar was received from storage tanks into rail tankers via steam plunger pumps (very old Russian design). Many times suction line got clogged and pump gave no/little flow. Perhaps that's why I am more concerned about line blockage, than the pump malfunction.

Regards.

Ajay

[markymaark]

The old saying "nothing sucks" comes into play here. The only things I can think of are:

1) The transfer lines overcomes the pressure differential driving force.
2) The vacuum pump is not getting the design vacuum inside the reciever tank when the tank is full perhaps forming a liquid type seal that the unloading pump cannot overcome. I agree with Breizh in post 4, cut the vacuum when unloading, I don't see any reason to keep it on.

[vicky123.]

I am really thank full to all of you to place your ideas.

Anyway just want to say that there is not a problem with the unloading pump or pumping lines (ie. suction or discharge line blockage) . As pe I concern the problemetic area is that due to vaccume in the suction reciver tank the liquid cant get enough npsh and thus the pump does not get suction uninteruptedly. beacuse when I stopped the vaccum pump and let the system come down to atmospheric pressure the reciver tank is empty out by the pump immidiatelly.

So please can some one suggest me such any ideas so that even if in full vaccum I can keep the reciver tank in atmospheric pressure.

Are any of you know any such process by which I can give the pump a normal suction pressure of flooded suction even if the suction reciver tank is under vaccum.

Edited by Art Montemayor, 26 January 2013 - 03:33 PM.
Post written in subscript font

[vicky123.]

Dear all

perhapes some of u misread the thing,

here the vaccum pump just make avaccum initially to give a start up to the system. when the reciver tank filled partially with the liquid comes from the railcar its comes to the unloading pumps suction & the pump delever it ti the desire tank. there is no funtion of the vaccum pumps beside initiate the syphoning action. yes, but at the time of unloading some time it sucks some air at the time of finishing every rail car so dose vaccum pump comes to act for an other session only that much.

[Art Montemayor]

650 mmHg = 0.85 atmospheres gauge pressure.

This is a relatively very small and weak driving force for a transfer operation. I would expect a driving force of at least 3.5 atm gauge.

In order to avoid the inherent small driving force that a vacuum system yields, Dow (in the very important and detailed file that Breizh submits) states the obvious and practical information:

• Caustic soda solutions are normally unloaded through the bottom outlet valve using either air pressure or a centrifugal pump to transfer the solution to a storage tank.
• If gravity flow to the pump inlet is used, the outlet valve in the piping from the transfer pump should be throttled to prevent cavitation on the suction side of the pump.
• Unloading through the dome using air pressure is satisfactory for 50% NaOH, provided the total lift from the bottom of the car to the top of the storage tank is not more than 30 feet.

I recommend you change this limited engineered design to something more practical and efficient – such as a pressurized air system as described by Dow Chemical Co.

[kkala]

Following are notes according to my understanding.
1. The vacuum pump develops vacuum of 600-650 mm Hg, that is absolure pressure 0.21 - 0.145 Atm a in the receiver.
2. Post no 10 seems to indicate that vacuum pump operation is necessary in the beginning to initiate siphon and in the end of unloading to restore siphon, if broken. Does vacuum pump operate in the intermediate time?
3. Post no 9 seems to indicate continuous operation of vacuum pump, is it true? (clarify between 2 and 3).
4. Flow to receiver (having much lower capacity than rail truck) continues when unloading pumps are operating.
5. No problem in the past, now flow of unloading pumps is improperly reduced after having unloaded 3-4 rail trucks (post no 6).
Difficult to diagnose reason, can you reply to above, or correct any imprecision for better understanding? It is difficult for me to interpret statement no 5 above. Problem seems to have appeared after some modification. Hopefully somebody will clarify (*), meanwhile I would try following.
α. Reduce flow to destination tank 6 (hence NPSHr) by partially closing discharge valves of loading pumps, assumed centrifugal.
β. If vacuum pump operates continuously, reduce vacuum in receiver to 600 mm Hg or lower to see consequence. If direct intervention on vacuum pump is not feasible, try to slightly open a manual valve of a thin vent pipe (permitting flow in both ways).
Of course more radical solutions proposed in previous post no 11 (and other posts) had better be investigated and applied in the long term.
(*) receiver had better not work under vacuum, assumed not under exact control during operation of unloading pumps.

Edited by kkala, 28 January 2013 - 03:42 AM.

[Art Montemayor]

Kkala:

You are stating that the conventional and practical method of unloading caustic soda as used successfully by Dow Chemical Co for countless years in the USA and elsewhere is "radical". I hope you have documented information to back that up because Dow Chemical - as well as all major chemical companies - would be very interested in learning about this.
Vacuum unloading (or "acid egg") of rail and tank cars is nothing new. I have used it to unload liquified compressed gases such as ammonia since the 1960's. What I consider impractical is using such a weak driving force to unload a solution of caustic soda that is known for its propensity to collect solids and has a relatively high density.

[kkala]

"Of course more radical solutions than curing existing situation, as proposed in  previous post no 11 (and other posts) had better be investigated and applied in the long term".

Art Montemayor, does above give more clear meaning to last sentence in post no 12 by kkala? (but it seems clear enough to me).

The conventional methods are not "radical"; their application on the existing scheme is a "more radical" solution (indicated in long term) than correcting what recently eliminated the so far satisfactory (yet unorthodox) operation. Query mainly asks about the latter, needing diagnosis.

Please also consider the note (*) at bottom of post no 12.