Designing Feed Pumps for Boiler Feed Applications

Designing Feed Pumps for Boiler Feed Applications FEATURED STORY

Upgrading boiler feed pumps to increase productivity at varying capacities can reduce the longevity of the pump if certain technical points are not considered. Pump manufacturers can preserve heavy-duty applications by accurately balancing axial thrusts, maintaining net positive suction head, and executing pump protection and control.

By Pablo Martinez- Moore, Commercial & Marketing Director, North Ridge Pumps.

Case Study

Feed pumps that use demineralized water are designed to operate in high-temperature and high-pressure conditions. Also known as boiler feed pumps, these units can be found in applications that produce steam using treated water. These pumps require uninterrupted operation and low inlet pressures. If the required feed water is not appropriately controlled, the demand from these pumps can result in cavitation.

A client’s boiler feed pump required alterations to increase productivity and operate in various capacities. The requested redesign of the pump suggested approximately 60M³H and a pressure of over 400M Head, matching the fluctuating steam demand during production. The clients’ boiler used demineralized water at temperatures up to 125°C and needed a higher output in steam. The requirements of operating the pump at various capacities meant that selecting units to accommodate a Variable Frequency Drive (VFD) is critical.

Pump manufacturers had to preserve the technical aspects of this heavy-duty application. The boiler pump needed to accurately balance its axial thrusts, maintain Net Positive Suction Head (NPSH) and execute pump protection to provide longevity for the units.

Boiler Water Treatment

Boiler feed is treated to prevent scaling by controlling alkalinity levels and conductivity of the water. Inside a steam boiler, a series of tubes are housed within a large, pressurized vessel. The feedwater pump continuously passes water through the metal tubes, heating the water to create steam. If the contents of the water are not controlled, it can lead to challenges such as blockage, reduced efficiency and quality of steam production, as well as corrosion in its tubes. Additionally, high conductivity creates corrosion when the boiler feed water presents a concentration of dissolved solids. When steam is produced inside a boiler, water particles are condensed and passed through a condensate return for reuse. Dissolved gases such as CO2 and oxygen can be present when the condensate cools, causing severe corrosion in boiler tubes, pipework, and other associated parts. To mitigate these risks, de-aeration devices, chemical scavengers and demineralization is required.

Balancing Axial Thrust & Thrust Bearings

Axial thrust appears at the front and back of the impeller on a multistage pump when the distribution of pressure is uneven and balancing the axial thrust is a common challenge. Each impeller stage is hydraulically balanced by inserting a bearing bush in the discharge casing to ease the force. The bush regulates the vibration, followed by a thrust bearing that absorbs the remaining forces in the pump casing. Alternative designs of multistage pumps may rely on a single bearing, located at one end of the pump casing or a motor bearing, to equalize the vibration. However, these pump designs cannot operate uninterrupted and are usually found in domestic or agricultural applications.

NPSH in Boiler Feed Applications

Boiler water is pressurized to ensure the feed can heat past its natural boiling point of 100°C and remain liquid when used with the pump. As mentioned before, the client’s boiler feed water must be heated to 125°C to produce steam. The condensate returns to the feed for reuse, but one common problem is cavitation. This is due to insufficient Net Positive Suction Head (NPSH) Available which is less than the NPSH Required (NPSHr). Engineers mitigate this challenge by implementing Low NPSH pumps in boiler feed applications to prevent premature failure of the components. The clients had an existing unit with an NPSHr of 3.5M, and the new model had a flow and pressure higher than this current model. The selected design’s NPSHr value was only 3.2M, even though the pump was operating at 60hz and at full speed- 3560 rpm.

Pump Protection and Control

The clients had requested that the boiler feed pump be designed to operate in various capacities to meet its fluctuating steam demand. Engineers achieve this by implementing an automatic recirculation valve to protect the pump from overheating, vibration, and cavitation. A recirculation valve automatically maintains a minimum continuous flow that benefits the pump’s longevity.

The valve can detect if the main delivery flow falls below a set value. This causes the valve to open its bypass, maintaining the minimum required flow delivery to the pump suction even if the output flow stops. Once the outlet flow increases the bypass closes. The flow is modulated and remains constant and it acts as a control valve. This avoids water hammering and pulsation. It minimizes these impacts because of the throttling effect which stabilizes the flow when the application is adjusted to a partial flow range. The modulated control enables the total rate of flow, and the minimum volume remains constant.

The clients were supplied with a horizontal multistage pump designed in materials to adapt to the demineralized water temperature of 125°C, delivering over 60M3H, at pressures over 400M. The multistage pump contains radial double-curved cast stainless steel impellers within the 5 stages. The stages are coupled with sealed O rings. The axial thrust from the impellers was hydraulically balanced at each stage. A discharge bush in the pump casing help to absorb vibrations. The remaining vibrations were absorbed by the main angular and ball bearings.

Variable speed operation can produce vibrations, the circulating current is prevented from flowing excessively with the aid of inner race bearings, its bearing balls or rollers. The pump has 4 bearings in total, 3 angular ball bearings and 1 roller bearing. The pump was supplied with the driveshaft and is supported by oil-lubricated bearings that are kept cool using an integrated oil-cooling fan. The unit was coupled to a 160Kw IE3 High-Efficiency motor suitable for variable speed operations with a bearing insulated at the non-drive end (NDE Insulated).

This unit was manufactured in under 14 weeks, packed to ISPM15 before shipping and was sent to the clients through sea freights.

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Shopia Ketheeswararajah is a feature editor contributing to Pump Engineer, Stainless steel World Americas, Hose and Coupling World, and other related print & online media.