Sealing Methods in Potable Water Pumps & Water Pump Casing Corrosion Allowance

Potable water is water that is safe for human consumption and suitable for drinking, cooking, food preparation and personal hygiene. Potable water originates from various sources but typically undergoes treatment to ensure compliance with drinking water requirements. Pumps and other system components used to distribute potable water are designed to maintain water quality and in addition are designed to minimize leakage and waste of the treated water.

To help achieve these objectives, pumps are equipped with shaft sealing systems that limit water from escaping where the rotating shaft exits the pump casing. Since potable water is not toxic or hazardous to people or the environment, limiting its leakage along the pump shaft is to maintain volumetric efficiency, limit any corrosion or damage to metallic parts (such as the pump baseplate) and to maintain a clean and dry environment around the pump. These objectives can typically be achieved with either a single mechanical seal or compression packing.

As illustrated in Image 1, mechanical seals consist of two highly polished sealing faces. One face rotates with the pump shaft while the other remains stationary in the pump housing. Springs and hydraulic forces keep these faces pressed together axially, with a very thin fluid film between them. This film lubricates the seal faces and minimizes friction while restricting leakage, typically such that it is not visible. Mechanical seals are widely used and must be selected to ensure compatibility with potable water regulations while providing reliable containment of the pumped fluid. Typical mechanical seal construction utilizes combinations of ceramic, composite, or metallic seal faces with elastomer secondary seals and stainless-steel drive components. The seal materials and all other wetted components and lubricants need to comply with regulations and incorporated standards for components in contact with drinking water.

Compression packing is a traditional shaft-sealing method that uses rings of braided material installed within a stuffing box surrounding the pump shaft. A packing gland compresses the packing rings against the shaft sleeve radially, creating a restriction that reduces leakage while still allowing a small amount ofwater to pass through. This controlled leakage serves to lubricate and cool the packing and the shaft sleeve, preventing excessive heat and wear. The packing requires periodic adjustment to maintain proper leakage rates and eventually must be replaced as it wears. When selecting packing materials, it is essential that the materials and any lubricants comply with regulations for components in contact with drinking water.

In either case (compression packing or mechanical seal), designers should consult the manufacturers and suppliers to verify that selected seal materials meet all applicable regulatory requirements for potable water service. Proper material selection ensures compatibility with potable water regulations while providing the durability and corrosion resistance necessary for reliable long-term operation.

It is important to select the seal flush plan based on the application requirements, such as temperature and pressure. A seal flush plan is the method by which liquid is circulated around the seal faces. Since potable water is typically clean and free of abrasive solids, the pumped liquid itself is often used as the flushing medium. The flush plan will help ensure proper performance and reliability by ensuring the flow is sufficient to remove any heat generated by the seal faces and provide liquid at a temperature and pressure sufficient to limit vaporization. Due to the typical cleanliness and temperature of potable water pumping, simple seal plans can typically be utilized that do not include coolers, filters, etc., but may be required in some applications. Some common seal flush plans include Plan 2 (dead-ended seal chamber), Plan 11 (discharge recirculation, as illustrated in Image 2) and Plan 13 (seal chamber recirculation to suction).

For more information on potable water pumps, refer to the Hydraulic Institute (HI)’s Pump Application Guidebooks at pumps.org/guidebooks.