Vertical vs Horizontal pumps for cooling tower, and lotsa others :)

[alwayslearning]

Dear All,

We are in concept development stage of a cooling water tower (fresh water). I am of the opinion that a cooling tower with extended basin to incorporate vertical turbine pumps (about 5 nos of 10,000m3/hr ; 35mts head) would be better. Other view in our group is that perhaps Horizontal pumps would be better because one could avoid the cost associated with Civil/structure for the extended basin.

I am sure those invovlved with the cooling tower/ and associated eqpt selection would have extensively done cost/ convenience/ maintenance/ reliability analysis on vertical vs horzintal pumps selection and associated merits etc.

Request all members to throw ideas, based on their experience, which is a better option. I realize that there aint a yes/no answer to this. Any guidance would be useful.

1) For the basin sludge removal, what kind of filtration recommended. Any specialist vendors's names pls.

2) For the cooling tower water treatment (fresh water) what checmicals are used. Any specialist vendors?

3) For the cooling tower blower, any particular recommendation/ requirements for the blower?

4) Most importantly, what is the expected uniterrupted interval for twoer maintenance works. For e.g. can we expect to operate the tower uniterrupted for 5 years.

Thanks for your replies

Regards

 

[SlideRuleEra]

Forced draft electric utility cooling towers universally (to my knowledge) use vertical pumps for circulating water.

There are no provisions for sludge removal, other than draining the basin and using a small front end loader (Bobcat) to "dig" it out. The basins are often designed with what looks like a boat ramp to allow easy access for cleaning. About 15 years ago, we started sloping the (interior) bottom of the basin to allow the sludge to be more easily pushed to the low spot for removal - this has been well received by maintenance personnel.

Cooling towers have a hard life - the evaporation of water means that a lot of "crud" gets left behind - this tends to make the water corrosive, potentially attacking both reinforced concrete and steel. One year of operation (again, for an electric utility tower), without a maintenance outage would be good.

One company name for water treatment is "Nalco"

http://www.nalco.com/ASP/index.asp

http://www.SlideRuleEra.net

 

[JJPellin]

I work in an oil refinery. We have five cooling towers. Only one of them uses a horizontal pump. This horizontal pump is so incredibly reliable that I have only seen it disassembled once in 17 years working there. The vertical pumps have trouble with cavitation in some towers even though the submergence is much more than the manufacturer requires. Subtle differences in sump design can result in pre-rotation of the fluid and cavitation even running at BEP or above. I prefer horizontal pumps in almost all services and cooling towers are no difference. But the cost of the pit will probably exceed the sump savings. All of our cooling tower fans are induced draft rather than forced draft. The key to reliability with the fans is oil piping that allows the oil to be filtered, flushed or changed from outside the shrowd. Clean oil is key to gearbox reliability. I would suggest you refer to Hydraulic Institute standards for sump configurations. You will probably end up with vertical pumps. I suggest careful selection of coatings (not just paint) to resist corrosion. I am not familair with sludge removal options, so I have nothing to offer there. The same is true of chemicals.

 

[Artisi]

My initial approach to this would be to first make some pump selections and associated pricing, ie capital costs, running cost etc, over the life of the station in association with the 2 types of installation. All other concerns would be the same.

Naresuan University
Phitsanulok
Thailand

 

[irishal]

ANother water treatment company is Betz Dearborne -

http://www.gewater.com/index.jsp

In my experience I have seen cooling towers taken out of service every two years for inspection and disinfection - I don't know if the disinfecition was a legal requirement to deal with potential for legionella.

Good luck
Irish Al

 

[jet1749]

I have 5 banks of cooling towers, all exlusively using horizontal split case double suction pumps retrofitted with mechanical seals. MTBF 3 to 4 years. All towers induced draught with mechanical gearboxes driven via external motor and driveshaft. Gearboxes updated to Amarillo dedicated cooling tower units with Howden composite fan assemblies and carbon fibre driveshafts and laser aligned. No failures in 5 years. Able to replace oil (synthetic)via extended drain and refill piping every year. No provision for sludge removal other than 6 monthly clean and disinfection to prevent legionella. Water treatment dosing with hypochlorite by Nalco. I have found horizontal split case double suction pumps very forgiving even with low npsha, and very familiar to maintenance staff.
regards
John

 

[alwayslearning]

Thanks to all for sharing the information. Looks like we have a pereference for horizontal pumps vs the vertical.

Jet, thanks for useful info on the vendors and eqpt. What is the capacity of each pump. Is space not a constraint in your plant? I looked at pump catalogue; 6.5mx3.5mx2.5tms (LxWxH). With a side suction side disch. we would have 3.5 mts + suction run + discharge check valve + BV + bend: all this would translate to say 6 mts?. So, rought a 6mx6m is used up in front of the tower. Perhaps this may be ok. I will check.

Is it not true that for horizontal pumps one may have to lfit up the tower to create sufficient water level.

You have a 6 monthly clean? Does that mean that out of the 5 towers, one is a spare, so that you can schedule for maint of one tower while other 4 serve the plant?


JJPellin,

You said "But the cost of the pit will probably exceed the sump savings.." Can you please elaborate what is meant by this.

Regards

 

[JJPellin]

Our horizontal pump is in a pit below the level of the ground in order to have positive suction head. Our towers are so large that it would be impossible to raise the tower. Our horizontal pump is driven by an 800 HP motor through a gearbox to the pump. The pit is probably 20 feet by 20 feet. Sorry for not converting the units to metric. I am pretty busy today. Our cooling towers are only shut down and cleaned out every 10 years or so. The older ones are constructed of Redwood with plastic distrubution grids. The newer ones are constructed of pressure treated lumber. The Redwood holds up much better but is not longer avaiable. We have the ability to shut down individual return lines so we can work on the fans two at a time with the tower still in service. Most of our towers have 6 to 10 fans. The fans are usually 16 to 24 feet in diameter running on top of a right angle gearbox with a long drive shaft coming out to the motor driver on the outside of the shrowd.

 

[jet1749]

The five banks of cooling towers I have are split between 3 separate business units on 1 factory site. The towers have between 2 and 9 cells each, and cells can be withdrawn from service on any tower for cleaning at will (some redundancy built into the system) Cleaning campaigns happen in spring and autumn and towers are cleaned on a cell by cell basis without the need to bring the whole tower out of service. Tower construction is timber on early units (1960's) and concrete on late ones (1970's)Static suction head on the pumps is between 4 feet and 6 feet dependant on individual tower (some pumps installed at grade and others in shallow pit) and individual pump flows of between 1500 IGPM and 3500 IGPM dependant on tower and pump type. Pumps are worthington simpson 8L2's/8NLA4's/10LA4's and SPP Thruststream TD25A's Space was not a real consideration at the time of construction. Fans are all around 10-11 feet diameter with 4-6 blades typically driven by 20-40 HP motors