Low NPSH Boiler Feed Pump

Boiler feed pumps operate in some of the most demanding applications. Their operation means units are tested to the limits of their design with operation being 24/7, their being low inlet pressures which can lead to cavitation, high temperatures and corrosive liquids.

A distillery contacted North Ridge who were looking to increase steam production by upgrading their boiler and replace a dated multistage pump with one of higher output of flow and pressure.

They were currently feeding the pump with demineralised Water at temperatures up to 125°C and needed a higher output of steam, meaning their existing unit needed replacement.

The duty point called for approximately 60M³H, and a pressure of over 400M Head. They wanted to operate the pump at a range of capacities to match their steam demand which fluctuates during production, meaning any unit selected needed to be suitable for operation via a Variable Frequency Drive (VFD) to alter the units speed to match their system needs.

Deionised Water and Boiler Water Treatment

Boiler feed applications frequently utilize treated or demineralized water. Treating boiler water is essential to prevent scaling by regulating its alkalinity and conductivity.

Uncontrolled water composition can result in blockages, decreased efficiency and steam quality, and corrosion of the tubes, which are challenging to replace and clean. High conductivity in boiler feed water, due to dissolved solids, can also exacerbate corrosion. Demineralized water is preferred for boiler feed applications as it is nearly free of minerals.

As steam is produced within the boiler, water particles accumulate, condense, and are recovered via a condensate return to be reused as boiler makeup water. When condensate cools, dissolved gases like CO₂ and oxygen can be present, leading to severe corrosion in boiler tubes, pipework, and associated components. These gases are removed using deaeration devices, chemical scavengers, and demineralization.

Given the heavy-duty requirements of this application, various technical considerations were necessary to ensure the selected unit would meet the demanded longevity.

There were 3 main technical points of concern in this application:

1. Balancing of axial thrust
2. Management of NPSH
3. Pump Protection & Control

Balancing Axial Thrust & Thrust Bearings

In centrifugal pump design, especially for units operating at high pressures or across varying duties, balancing axial thrust is a crucial consideration. Axial thrust arises from uneven pressure distribution at the front and back of an impeller or across a series of impellers in a multistage pump.

In heavy-duty applications, this thrust is managed by hydraulically balancing each impeller stage, using a bush in the discharge casing to absorb the forces, and installing a thrust bearing at the discharge end of the pump casing to handle any remaining forces.

Inferior multistage pump designs may have bearings only at one end of the pump casing or rely solely on the motor bearing to balance these forces. These designs are typically used in agricultural or domestic applications and are unsuitable for 24/7 high-pressure boiler feed applications, which are critical in nature.

NPSH in Boiler Feed Applications

A common problem in boiler feed applications is cavitation of the pump due to insufficient Net Positive Suction Head (NPSH) Available being less than the NPSH Required (NPSHr) by the boiler feed pump. This issue arises because the high-temperature boiler feed water (125°C) is above the natural boiling temperature of water (100°C). Boiler water is pressurized to allow it to remain liquid without vapor or steam.

Low NPSH pumps are essential to prevent premature component failure in these applications. The client's existing unit had an NPSHr of 3.5 meters. Despite the higher flow and pressure of our model, it had an NPSHr value of only 3.2 meters, even at 60 Hz and full speed (3560 rpm).

Automatic Recirculation Valve for Pump Protection

An automatic recirculation valve maintains the safe minimum continuous flow required by the pump. This valve ensures sufficient flow for effective cooling when the pump speed is reduced, protecting the unit from overheating, vibration, and cavitation. It operates by detecting when the main delivery flow falls below a set value, causing the valve to open its bypass and maintain the minimum required flow to the pump suction, even if the output flow stops. When the flow rises above the set range, the bypass closes, increasing the outlet flow.

The modulation of flow by the valve avoids water hammer and pulsations, stabilizing flow in partial flow ranges. Consequently, the sum of delivery flow and minimum volume remains approximately constant. Typically, an automatic pump protection valve operates between 40% to 100% of volume flow. In closed-loop applications, with suction and a recirculation line back to the tank, careful consideration should be given to material selection to minimize corrosion under high-velocity conditions.

Summary: Benefits of an Automatic Recirculation Valve

1. Flow is measured and modulated.
2. The valve acts as a check valve, preventing water hammer.
3. Bypass operation is automatic.
4. Bypass pressure and flow are reduced without cavitation.

We recommend fitting the pump with an automatic recirculation valve to ensure longevity of operation.

Outcome

North Ridge specified and supplied a horizontal multistage pump designed in materials to accommodate the deimineralised water at temperatures up to 125°C, delivering over 60M³H, at pressures over 400M.

The multistage pump consisted of 5 stages, with the radial double curved cast Stainless Steel impellers housed within the several stages sealed by O Rings, held in place via external tie rods.

The axial thrust from the impellers was not only hydraulically balanced at each stage, but a discharge bush in the discharge casing helped absorb axial forces, with any remaining axial thrust absorbed by the main angular and ball bearings.

The unit contained 4 bearings in total for heavy duty operation which consists of 3 angular ball bearings and 1 roller bearing. The pump was supplied with the driveshaft supported by Oil lubricated bearings kept cool by an integrated oil cooling fan.

The unit was coupled to a 160Kw IE3 High Efficiency Motor suitable for variable speed operation with a bearing insulated at the non-drive end (NDE Insulated), preventing circulating current from flowing via the inner race of the bearings, its bearing balls, or rollers to the motor frame which can occur during variable speed operation due to the harmonics present at various frequencies.

The unit was manufactured in under 14 weeks, and packed to ISPM15 before being shipped to the client.