Mckenzie Corp- Boiler Feed Pumps
Posted in the Top Stories Forum
#1 Aug 27, 2014
Boiler feed pumps are an important part of any boiler operation. They control the amount of water fed to the boiler and the manner in which it is fed.
Centrifugal - Continuous
Turbine - Intermittent
In order to properly select a boiler feed pump five key points must be considered:
Will the pumps operation be continuous or intermittent?
This is an operational question and is often answered by the type of level control found on the boiler that the pump will be servicing. As a general rule of thumb, boilers with a capacity of 10,000 lbs./hr. or less utilize a float type switch that starts and stops the boiler feed pump to satisfy a predetermined water level within the boiler. This is a classic intermittent operation.
Boilers with capacities exceeding 10,000 lb./hr. typically employs a modulating feed water regulator and will continuously feed water to the boiler at various rates depending upon the water level in the boiler.
By knowing which operation you are to satisfy, you can determine which pump design is best suited for your application. As a general rule of thumb a turbine pump is used in an on-off situation and a centrifugal pump is used for continuous operation. But remember, this is a general rule and is some cases a centrifugal could be used for an on-off application and a turbine for continuous.
What is the temperature of the water being pumped?
It is also important to know the temperature of water you intend to pump. Most pumps can usually handle 215 oF to 230o F, other pumps are available that can handle higher temperatures by using external water-cooling. Keep in mind that a deaerator pump must be able to handle higher temperatures because they operate a 5 psi or 227o F.
What is the required capacity?
How much water you intend to pump is dependent upon the evaporation rate of the boiler the pump will service. A safe figure for an on-off application would be 2 times the evaporation rate of the boiler. With a modulating level control, a factor of 1.3 times the evaporation rate plus recirculation is recommended.
What is the desired discharge pressure?
When you pump directly into the boiler you will need to overcome the pressure in the boiler as well as any piping losses. You can chose the right pump by looking at the pump curves to determine which will accomplish this task. Should you have a modulating valve in the discharge line, the minimum you will need to add to the boiler operating pressure will be 20 to 25 lbs. Make sure that the pump can handle the pressure along with the flow rate needed. With an on-off level control the pumps should be designed for the relief valve pressure.
What is the NPSH or net positive suction head required?
This is the last piece of information that you will need. This is the minimum absolute pressure at the suction nozzle at which the pump can operate. To avoid pump cavitation, the NPSHA of the system must be greater than the NPSHR of the pump. In other words, the available NPSH must be higher than the required. We have always sized our deaerator stands to be two feet higher than the NPSH needed for the pump selection. Remember, the water level in the storage tank adds to the safety margin.
Visit Us: http://www.mckenziecorp.com/
Great Yarmouth, UK
#2 Aug 28, 2014
That is very informative and useful. There is small word of warning I would like to put forward. I once worked at a very large laundry factory. The three boilers were serviced by two steam driven water pumps. The engineers used one pump for week then the other one for a week. One morning the pump they were using broke a ring. they immediately switched to the other pump and that broke a ring within an hour. Both pumps had to be stripped down and the whole factory came to a standstill for the rest of the day. When I was running a plant with two pump I would use the same one for most of the week and the other one on the Friday. So I knew both pumps worked and when one wore out we had a spare in good condition
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