Pump Seal flush coolers 1

[sgoel]

We have many pumps having seal flush coolers. Cooling media used is Cooling water on shell side. These coolers are verticle cylindrical with process media on spiral tubing inside.Cooling water return temperatures could be as high as 60 deg c.

We find repetitive fouling on the surface of the tubings. On analysis we found about 70 % of this matter to be Ca and Mg salts and phosphates.

a)whether simillar problem is also faced in other places also?
b)What is the possible corrective action in such instances.

Thanking you in anticipation,

best regards

 

[FRChem]

If these are the so called "gland seal flushing" coolers, then I have thefolloeing to say;
Seal flushing is normally taken from dedicated system (or from pump discharge), returned and cooled in the above mentioned coolers and then diverted into seal system. What I'm familiar with is that the cooling water should be temperted cooling water rather than plant regular cooling water. TCW is better in quality and suitable for higher operating temperatures, 50 - 60 °C, in order to avoid such problems. TCW is anormal cooling media that is used with pumps.

 

[Phil100]

It appears that you are getting deposition of hardness salts from your cooling water onto the exchanger surfaces. This is a widespread problem the world over...companies such as Betz Dearborn and Nalco make a living curing such problems by having technical service people sample and treat cooling and heating systems with chemicals! 60 C is generally accepted as the temperature above which deposition occurs.

I expect that if the bulk cooling water return temperature is 60C then the film temperatures on the exchanger surface might well be above 60C leading to the deposition problems.

As a rule the water used in heat exchangers should be treated to prevent hardness salts from depositing for example with chelating agents or the hardness salts removed by ion exchange or similar.

So the cure is to treat the water or alternatively increase the flow such that the film temperatures remain below say 50C to discourage deposition.

 

[abcmex]

I supposse the deposits are on the CTW side. If yes:
I agree with Phil100 that probably they are caused by the hardness of the water.

In the cooling water usually you concentrate the hardness of the plant water (make-up) to 4-5 times, but do not jump to conclusions, this water is usually chemically treated to avoid precipitation.

1. The problem may be that to determinate the level of chemical program your supplier may have not know that the water could go up to 60 deg. and therefore there is insufficient protection. Check this with them (Nalco, Betz, etc.)

2. You do not state the process side Temperature range but I believe is hot enough. If you increase the flow of CTW perhaps you will overcool the process side. Even if you can I suppose there will be always contact with high temperature fluid and the problem will start again. So the second possible solution is a secondary cooling system between the seal coolers and the CTW loop filled with an adequate fluid. see link below

http://www.sentry-equip.com/CWIS_products.htm

3. other posibility is to use an air cooler

http://www.flowserve.com/seals/literature/lit_FSD197_Airfin_data.pdf

rgs

 

[longeron]

Water quality is one of the easiest and first things to look at when rapid repetitive scaling occurs in seal flush heat exchangers.

Four seal flush plans that immediately come to mind that use heat exchangers; Plans 21, 23, 52, and 53

21- Recirculation from discharge through a heat exchanger to the seal chamber

23- Circulation of fluid in the seal chamber through a heat exchanger. This plan generally employs a pumping ring to help develop head to promote circulation, as well as a bushing in the stuffing box to isolate the cooled fluid from the pumpage

52- Buffer fluid circulated through a tandem seal. The heat exchanger is in the buffer fluid reservoir

53- Barrier fluid circulated through a dual seal. Again the exchanger is in the reservoir.

I'm guessing that you're using the Plan 21 at the moment. The biggest advantage that this plan has include the fact that operators have little to do with the operation of the seal. The higher pressure fluid off discharge shoots right on through the exchanger into the seal chamber and back into the volute. Ideally this plan shouldn't have any valves in the flush tubing, there may be a need for an orifice depending upon the desired flush rate and pressure. The disadvantage is that this plan has the highest dT. The larger the difference in temperature between the flush fluid and the cooling water the faster the cooler will foul. This may be part of the problem you describe.

Plan 23 avoids the large dT because it circulates the same fluid constantly. The seal chamber is isolated from the volute by a bushing so that it is much cooler than the fluid at the pump's discharge. Lower dT- longer seal flush exchanger life. However there is a price- Operators and millwrights are more involved with the seal and flush plan. This plan requires that the cooler is installed a certain way so that the cooled fluid drops- helping to circulate the flush... Thermosyphoning is the term most seal guys will use. Heat rises, cold sinks. Tubing runs should be short within the guidelines of the cooler height requirements, large diameter tubing and large radius bends to decrease friction losses. This is why most seals that use a Plan 23 have a pumping ring of some kind. Even when the pump is not running, the thermosyphon effect will help keep the seal chamber cool. If the seal chamber, exchanger, and tubing are not properly vented at start-up this flush plan may not work properly. Bubbles tend to keep the fluid from circulating, a high point bleed is required... depending upon the pumpage you may need to make arraingments to sewer or recover small amounts of process fluid.

Buffer fluid circulation is generally the same idea as a Plan 23. This time the fluid is circulated between two seals at a pressure lower than the stuffing box and, possibly higher than atmospheric. The idea is containment of the inevitable leakage across the primary seal faces. Generally the reservoir is piped to a flare or knock out system... The same problems of a Plan 23 apply here.

Barrier fluid on the other hand is pressurized higher than the process fluid in the stuffing box. This means that the barrier fluid wets both sets of seal faces, the process fluid is isolated as long as the barrier fluid remains pressurized and as long as there is barrier fluid. Operations should monitor the fluid level in the reservoirs (Both 52 and 53).

Talk to your favorite mechanical seal vendor about the flush plan you are using ask if the heat exchanger is piped or installed correctly. I'm sure that they would be happy to discuss alternatives or how to correct any installation or operations problems.

 

[bewdley]

I have occasionally used spare condensate from various sources to flush mech seals. This is easy when you have something like an evaporator, but it might be worthwhile to consider condensing steam vents to get a perfectly clean liquid. The demineralized water you get is what yous should feed to he system, at least as a makeup.