Most boiler feed water pumps (BFW) around the world have a centrifugal pump design. This pump design uses multiple stages of impellers to create the pressure needed to move process water through a boiler loop. Customers often use an API 610 BB3 pump design for this service, as it is common for boilers with a 1,000 HP to 2,500 HP power range and a temperature range from 200°F-400°F. Other centrifugal multistage pump designs can be used for this service range, but for this article we will concentrate on the API 610 BB3 design, which is an axial split case multistage centrifugal pump.
Pump Construction Material
Boiler Feed Water is often deionized which can make the water corrosive, especially at higher temperatures. Avoiding the use of high carbon-based steel or iron in the pump’s construction, especially in the pressure containing components, is generally advised. The use of 300 series stainless steel may not be necessary and can result in unnecessary costs to the pump, as it would increase material costs; this becomes evident when considering the amount of material that is involved in a BB3 pump design. Thermal expansion properties of 300 series stainless steels can also cause problems with the pump assembly at higher temperatures that are commonplace in BFW services.
API 610 Material Code C-6 is therefore a common material option for API 610 Annex H. C-6 material is a 12% Chrome material (ASTM A487 Gr. CA6NM for the Pump Pressure Casing) which has a high corrosion resistance to most boiler feed water qualities. The material has an attractive cost vs benefit ratio for most BFW systems and its thermal properties are ideal for higher temperature applications. As the pump is continually subjected to process water, it is best to conduct a water sample analysis on the water that is to be used in the boiler to ensure that the most appropriate construction material for a pump is selected.
Mechanical Seal Design & Flush System
Single cartridge mechanical seal designs are commonly used for BFW services in lieu of double tandem seal designs. Single seal designs have proven to be reliable if they are configured properly for the application. As water is an extremely poor lubricant for the mechanical seal faces, especially if the water has an elevated temperature. Mechanical seals generate frictional heat when in operation. Special attention should be given to the mechanical seal face materials and the cooling flush system that is used to cool the seal.
For most BFW fluids, a carbon face material can be used on the seal rotating face in conjunction with a Silicon carbide face material on the stationary. If there is a high content of solids or slag in the BFW system it can cause failure to the carbon seal face. It is therefore essential to manage the boiler feed water quality and limit the amount of solids entrained within the water loop.
It is important that a mechanical seal is used with an efficient seal flush system. The flush system should remove heat from the seal chamber environment, which arises due to the mechanical seal face friction; the cooler the water is around the seal faces, the more heat can be removed from the seal face friction dynamics. Without efficient heat removal from the seal environment, the seal will overheat, fail and cause the process water to leak to the outside environment. A cool running seal, is a happy seal. To attain this, the incorporation of a bypass flush with a heat exchanger is recommended. Two flush systems come to mind: the API/CPI Plan 21 Flush and a Plan 23 Flush both provide efficient heat removal for the water bypass seal flush.
The Plan 21 is configured to bypass a small amount of the boiler feed water in the pump from a high pressure point on the casing and bypass it through a heat exchanger where heat is removed. The sub cooled water then goes into the seal chamber where it can efficiently remove heat from the seal faces. The water then recirculates back into the pump fluid process. There is, however, a dark side to the Plan 21. If the water quality has even a slight solids content, the coil within the heat exchanger can become foaled quickly and the bypass flow is reduced. This occurrence results in a loss of cooling and causes premature seal failure. The limited retention time of the water in heat exchanger on the Plan 21 can also affect the temperature differential throughout the exchanger. The flush flow rate can be very difficult to control, which can result in a less than desired temperature delta across the exchanger.
It is recommended that the Plan 23 be considered over the Plan 21. The Plan 23 flush system does not take bypass from the high-pressure side of the pump. The Plan 23 functions by partially isolating the water within the seal chamber environment from the rest of the process water via a throat bushing in the seal chamber bore. Recirculation of the water is accomplished by using a seal with a pumping ring feature, which provides just enough kinetic energy to the flush to circulate the water through the heat exchanger and back to the seal. This allows the isolated water within the seal chamber to constantly circulate, providing a much greater cooling effect on the mechanical seal faces. A throat bushing in the seal does an efficient job of keeping process solids from migrating into the seal chamber. Plan 23 flush system has proven to be a very reliable flush plan in these BFW applications with the subject temperature range.
Subjecting all of your fluid properties and pump hydraulic variables to a mechanical seal subject matter expert, to get a recommendation on the seal design and flush system, is highly advised.
Wear Ring Considerations
Wear rings are an essential component of the subject pump’s design and application. Typically, the pump impeller will have wear ring surfaces that are either renewable or integral; the casing will have stationary wear rings that mate with each impeller rotating wear ring. The radial running clearance between the two rings affects the internal recirculation of the process water in the pump. The amount of fluid that recirculates across the ring clearance has a direct effect on the pump’s efficiency.
In a construction using C-6 material there will typically be 400 series stainless steel wear rings in the pump. The pump manufacturer will harden the 400 series rings so that there is at least a 50 Brinell hardness differential between the impeller and casing wear rings which helps prevent galling between the two rings during operation. Running clearance on these rings can be quite high at an elevated temperature. API 610 recommends running clearances greater than the pump OEM manufacturer’s clearances, especially as process temperature increases. High running clearances can provide improved reliability over time. As dry running a pump with metallic rings installed can cause catastrophic failure to the pump in less than one minute, special measures should be taken to prevent dry running the pump when metallic wear rings are installed.
Today there are a number of options on wear ring materials for customers to choose from in lieu of the traditional metal-to-metal ring combinations. Over the past two decades there have been many composite thermoplastic wear ring materials that have been introduced to the market. These composite materials are typically used on the stationary casing wear ring component. The composite material is installed inside of the metal ring as shown here. Composite material is also recommended for the stationary center bushing component on the API 610 BB3 design. Most composite rings can operate up to 500°F, allowing it to operate in most Boiler Feed Water applications.
With a composite wear ring on the stationary ring, running clearances can be taken to much tighter values, when compared to a metal-to- metal ring clearance. For example, the API 610 requires a .020” diametrical running clearance on a ring that is 8.000 inches to < 9.000 inches in diameter. Composite wear rings can accommodate a .011” diametrical running clearance on that same ring dimension. This ultimately improves the efficiency of the pump. In addition to its ability to run a tighter clearance, the composite ring also has a low coefficient of friction. This low coefficient prevents excessive heat buildup during an upset or temporary dry run condition. There are many case studies in the industry which show customers who have prevented a catastrophic and costly pump failure by having composite stationary wear rings installed on their pumps.
Composite rings come at a price. They can often add $15,000-$25,000 to the cost of a large API 610 BB3 pump. I will add that not all composite wear rings materials are created equal. Some will have different radial thermal expansion properties than others. It is encouraged that a pump subject matter expert conduct research before one decides on the type of composite ring material to use. Finally, caution is strongly recommended if using composite wear rings material in fluids with high solids content. If there is heavy scale or high solids in the boiler feed water, it is best to stay away from the composite ring materials. The thermoplastic base material can wear quickly in a high solids environment.
Composite wear rings (black) and center bushing backed with a metallic ring on an API 610 BB3 pump design.
Bearing Design and Cooling
The API 610 BB3 pump design comes in many different bearing configurations. The thrust bearing is located on the non-drive-end of the pump, and it constrains the hydraulic axial thrust load generated within the pump, during operation. Depending on the amount of thrust being generated, and the horsepower of the machine, the pump manufacturer will be able to determine if a ball (roller) bearing, or more complex tilted pad bearing design, is required. The drive end radial bearing can come in ball (roller) bearing or sleeve bearing design. The customer can often request what configuration he/she prefers on the drive end (motor end) of the pump shaft. The sleeve bearing on the radial drive end side of the pump is more typical on the subject pump design.
I often have customers asking what I recommend for the best oil grade on ring oil applications. Ring oil lubrication method is when the bearing housing has an oil sump and a forced lubrication system is not in place. For pumps with an oil sump and ring oil configuration, pump manufacturers will often recommend an ISO Grade 100 oil for process applications that exceed 180°F. One should review his/her pump man
I also get asked at what process water temperature to install an immersed cooling fin in the oil sump to pull heat out of the oil sump on oil ring ball (roller) bearing configurations. Avoiding the use of any cooling coils to cool the lubrication oil on a boiler feed pump is generally what I recommend. I have seen plenty of cases where the cooling water is circulating through the coil when the pump is turned off. This causes the oil and bearing housing to cool below the dew point of the ambient air. This in turn results in water condensation forming on the interior of the bearing housing. If the condensed water gets into the oil, it compromises the oil quality, resulting in bearing failure. Water contamination is the Achilles heel of oil quality. The use of a synthetic oil, in lieu of a cooling coil, is often recommend. With the right synthetic oil selection, a system will operate with process fluid temperatures up to 700°F without cooling the bearing lubrication. I recommend an external forced oil lubrication system, or an oil mist system as the primary lubrication method on this type of pump BFW application. It is important that pump OEM recommendations are followed closely to ensure that the best lubrication path is provided for the machine components based on the system variables. Selecting proper design options on boiler feed pumps is essential for long term optimal reliability. An API 610 BB3 on boiler feed water service can easily get five or more years of mean time between repair (MTBR), if properly configured, installed and operated within its design parameters and manufacturer’s recommendations. Working with a pump and service provider who understands the challenges involved with boiler feed water service can also be highly beneficial.