17 Steps You Need To Know To Find The Best Boiler Feed Pump
March 4th, 2020
Finding the right boiler feed pump for your boiler systems is crucial. Not all boiler feed pumps are created equal, and not all can do the job fit for your industrial application. To maximize your system’s capabilities, you need to find and assemble the right parts.
Boilers are pressurized tanks that use heat to vaporize water and turn it into steam. The produced steam has many industrial applications such as heating homes, heating water, power
generation, sanitation, and is used in a variety of other industrial applications.
Boiler systems are elaborate machines that use different stages to turn water into steam. One of the first steps in the steaming process is feeding water into the machine. To do so, a boiler must have a boiler feed pump.
Boiler feed pumps are specific pumps that carry the water into a steam boiler. These pumps are usually high-pressure units. Freshwater is supplied, which becomes steam. Not all of these vapors will be used so the condensed water is recycled back using a condensate return system. The water goes through the boiler feed pumps again and the cycle repeats itself.
A boiler feed pump is a crucial part of any boiler system. All processes begin with the feedwater entering the boiler system. Thus, finding the best fit for your boiler can ensure safe and enduring operation. Following these steps can help you find the right boiler feed pump for your system.
1. Find out the control method to be used.
Determining the control method that will be used for feedwater delivery will help you select what type of boiler feed pump you need. There are two common control methods: on-off control and modulating feedwater control. If you use an on-off control, you need a very tough boiler feed pump that can last the wear and tear of sudden water surges. Possible water surges are less of a concern with a modulating feedwater control.
2. Calculate the base flow rate.
A flow rate is the amount of water that comes out of the tap in one minute. In boiler systems, you need to have a steady input of feedwater. Applying pressure can help keep the flow rate of the feedwater steady. In order to do that, you need to find out the base flow rate first.
A good way to determine the base flow rate is through this formula: boiler maximum capacity horsepower x 0.069 x C. The value of C is based on how your pump will cycle: either in intermittent mode or in continuous feed mode. If your pump will have an intermittent mode, you can use the value of 1.50. If it’s continuous, it’s 1.15.
After getting your base flow rate, you can calculate for your total flow rate. However, there are some factors you need to consider, too.
Disclaimer: Carver Pump is an American-made pump company, and not a boiler company. All calculations related to the boiler, such as its size and shape, should come from the boiler company you’ve chosen.
3. Consider the continuous boiler blowdown flow.
The continuous blowdown flow works to remove total dissolved solids (TDS). This is important to ensure that no impurities can enter the boiler that could add to the wear and tear of the system. Although not a necessary step, it’s good to add the continuous boiler blowdown flow to determine what type of boiler feed pump you need. To do this, add 10% of the pump’s efficiency flow rate.
4. Check the bypass flow rate.
If your desired control method needs a bypass recirculation line and valve, you need to add the bypass flow rate to get the total flow rate. Some control methods like variable-speed drive (VSD), continuous bypass system, and a controlled bypass system can affect your total flow rate. All these control methods allow maintaining a minimum flow to avoid operating close to the shutoff head. Each bypass flow depends on the minimum flow requirement of the pump. It’s usually 10% to 20% of the pump’s flow. Just a reminder, always check with the manufacturer or review the pump’s tech manual.
5. Compute for the total flow rate.
Now, that you have the base flow rate and other factors like the continuous boiler blowdown flow and bypass flow rate, you can compute for the total flow rate. The total flow rate is the amount of water that passes through the system in a given time.
6. Determine the feed pump’s base head.
After getting the total flow rate, you now need to get your pump’s total dynamic head (TDH). The TDH is the total amount of pressure when water is flowing in the system. Just like how you computed for the total flow rate, you start with the feed pump’s base head. Here are the formulas to get your pump’s base head:
- At the duty point flow rate (in feet): (boiler operating PSI) X (2.31) X (1.03) / (liquid-specific gravity)
- At shutoff head (in feet): (pressure relief valve PSI) X (2.31) X (1.03) / (liquid-specific gravity)
7. Include all suction piping head unit measurements.
Another crucial part of the boiler is the deaerator. It takes oxygen and carbon dioxide from the feedwater so that pure water enters the boiler. Aside from these elements, the deaerator also removes the impurities from the feedwater. Doing so extends the boiler’s life.
Calculate the gauge pressure of the deaerator tank, elevation head from the waterline to the centerline of the pump’s lowest impeller, and all of the suction line friction losses. In getting the elevation head, make sure you are measuring from the suction side.
8. Include the discharge piping system head components.
Another factor that you need to get your feed pump’s TDH, you need to compute for all the friction losses on the pump’s discharge side. Include the elevation to the inlet relative to the discharge.
9. Compute for the total dynamic head.
Add all the measurements of the head components: the base head, suction side piping head, and piping system head on the discharge side. Make sure that the base head includes the safety factor.
10. Determine the shutoff head.
Aside from getting the correct head at the required flow rate, you also need to consider the shutoff head. To determine the head at zero flow, the head must be at the safety valve setting and then add three percent.
11. Identify the temperature at the boiler feedwater tank.
Selecting your boiler feed pump also relies on its capacity to withstand different types of temperature. Since the boiler feed pump’s task is to carry feedwater from the feedwater tank to the boiler, the pump itself will be exposed to different temperatures.
12. Calculate for the net positive suction head available (NPSHa).
The net positive suction head available (NPSHa) is a way to measure how close the water is to flashing. Usually, the NPSHa can be estimated when the system is being designed and constructed. You can also calculate it using this formula:
- NPSHa = (the absolute pressure in the feed water tank) ± (elevation of minimum water level in the tank above feed pump) – (vapor pressure of water in feed tank) – (suction line friction loss)
13. Review available boiler feed pumps that can accommodate your total flow rate, TDH, and NPSHa.
Make sure to consider the curves needed so your pump can meet the calculated conditions. It
is a good idea to choose a pump operating at or to the right of the pump’s Best Efficiency Point (BEP) if your maximum flow rate has been used to size the pump. Boilers often operate at a
percentage of their max flow rate, and so having a pump that can operate efficiently at these
reduced flows can ensure longevity of your machines.
14. Check that the shutoff head can also be accommodated.
After selecting the boiler feed pump that meets the desired conditions, make sure to consider the shutoff head as well. The ideal shutoff head must not be less than three percent above the safety valve setting.
15. Check net positive suction head required (NPSHr) versus NPSHa.
When it comes to installing the boiler feed pump, the net positive suction head required or NPSHr should be checked against the NPSHa. Also, make sure there is some margin between the NPSHa and the NPSHr (required). At the pump manufacturer’s published NPSHr value, the pump has already experienced a 3% drop in the head, which means the pump is cavitating. This is why providing some margin between the available and required net positive suction head is important. Each NPSHr and NPSHa is unique. Thus, it must be checked (and configured) whenever a boiler feed pump is installed.
16. Double-check material compatibility.
One more factor to consider in choosing your boiler feed pump is its compatibility with the chemicals and the temperature it will handle. The goal is to have a boiler feed pump that can last a long time. Finding the right one that can handle the various chemicals and in different temperatures is the key to longer-lasting equipment.
17. Find the brand that you can trust.
Last and not least is to find a brand that you can trust. There are many options for boiler feed pumps, but there are only a few brands that you can rely on. Aside from calculating the specifications and requirements for your boiler feed pump, read reviews and recommendations on brands that you can trust.
A company that delivers quality products is Carver Pump. With over 80 years of experience, Carver Pump has built a reputation for reliability and stability. It’s been recognized as a company that produces leading centrifugal pumps and designing pumps that meet high-standards and demanding engineering specifications in the world.
Carver Pump’s quality management system is certified by Intertek to conform to ISO 9001:2015. Our commitment to quality includes not only our hardware, but also superior customer service, leading-edge R&D, and continuous improvement in everything we do.