Upgrade from pump packing to mechanical seals

The environmental performance of products and processes in all indus­trial sectors increasingly is cause for critical inspection, with sustainability, conservation of natural resources, and reduced environmental contamination concerns influencing equipment design and selection.

Many industrial processes can be addressed to improve sustainability and minimize envi­ronmental impact, while at the same time maintaining or reducing operating costs. Implementing energy-efficient and environ­mentally friendly processes and technologies should be embraced as a priority at the compo­nent, process, and system levels.

One aspect of these processes is mission-critical rotating equipment, and specifically cen­trifugal pumps, which represent a significant proportion of the equipment found in industrial operations. One vital component of a centrifu­gal pump is the seal around the rotating shaft that passes through a stationary pressure cas­ing or housing. The seal contains the liquid or gas from escaping to the environment.

Sealing systems help maintain acceptable pump efficiency, reliability, energy consump­tion, water usage, and emissions control. These factors can materially facilitate achiev­ing total-lifecycle cost-reduction and sustain­ability objectives. Sealing performance can be improved for centrifugal pump applications by upgrading from traditional compression packing to mechanical seal technology.

When sealing a centrifugal pump, the objec­tive is to allow the rotating shaft to enter the wet area of the pump without large volumes of pressurized fluid escaping. The pump dis­charge pressure forces the fluid back behind the impeller, where it is induced to exit by way of the rotating drive shaft. To minimize leakage, a seal is needed between the shaft and pump housing to contain the pressure of the process being pumped and withstand fric­tion caused by shaft rotation.

Cutaway of a mechanical seal. Courtesy: John Crane Inc.Compression packing is the traditional means to seal centrifugal pumps, going back more than 100 years. Also referred to as gland packing, it is a braided, rope-like, and lubricat­ed material packed around the shaft in rings, physically stuffing the gap between the shaft and the pump housing, within a stuffing box.

Water leakage and consumption

For compression packing to work, some leak­age must be maintained to lubricate and cool the packing material. Therefore, packing rings allow for an adjustable, close-clearance leak path parallel to the shaft axis. As the packing is used, however, some of the lubricant that is embedded into the packing is lost, reduc­ing the packing ring’s volume. The pressure squeezing the rings together is also reduced, increasing leakage.

Periodic adjustment of the packing follower brings the pressure back into specification and controls the excess leakage. In today’s world, however, this maintenance is not always being done at required intervals or adjusted correctly. As the number of cen­trifugal pumps incorporating the use of com­pression packing decreases, training for and understanding of packing maintenance has waned.

Consequently, under-tightening and over-tightening of packing rings is a prevalent and growing misapplication of centrifugal pump maintenance, with critical consequences to both water consumption and energy draw.

Under-tightening results in too much leak­age. Already, when properly adjusted, packing leakage can amount to gallons of liquid leaked per minute. This can be either aqueous solu­tions comprised of varied benign or caustic chemical compositions, or particles in suspen­sion or slurry, depending on the process.

The heavier the suspension or slurry con­tent in the pumped liquid, the more water is needed to get packing to work reliably. Typically, a clean external flush is piped into the stuffing box through a lantern ring, which keeps the packing lubricated and cool while flushing abrasives and chemicals.

Normally, some portion of the leakage is released continually into the atmosphere. Under-tightening of the packing rings and use of external flushes increase this atmospheric release proportionately, along with environ­mental impact potential.

Friction and parasitic energy draw

Friction is always present in centrifugal pumps with compression packing, due to the large surface area of the packing rings in contact with the shaft. Over-tightening pack­ing rings restricts leakage flow, increases friction between packing and shaft, and gen­erates excessive heat, which degrades the packing. Increased friction also wears the shaft prematurely.

From an energy consumption perspective, the additional friction of the packing gripping the shaft creates increased drag, requiring more drive power to turn the shaft. It is that drag that leads to additional, significant para­sitic energy draw. Thus, the friction-induced energy draw is critical to the energy efficiency of the compression packing.

Moreover, friction is not the only factor influ­encing energy usage associated with com­pression packing. When examining the energy draw component of a total lifecycle-cost analy­sis related to compression packing use in cen­trifugal pumps, another consideration factor is the external flush piped into the stuffing box, as this pressurized water or fluid needs to be moved from a source location to the packing, requiring a pump that draws electricity. Also in some industries where compression packing is more commonly used, water added via the packing flush to maintain a clean environment around the pack­ing needs to be taken out later. Removal of this water requires energy, typically through boiling and via the pump heat soak, with energy transferred from the hot metal of the pump to the fluid within the packing chamber.

These energy draws typi­cally are not measured directly. Instead, current and voltage fluctuations used by the pump motor are assessed under varied operating conditions to determine how much power is being con­sumed by parasitic influences, which enables packing energy deficiencies to be identified.

Mechanical seals support efficiency

The mechanical seal is an alternative to com­pression packing that resolves many of the sustainability and environmental-impact issues inherent in compression packing. The mechani­cal seal requires much lower water and energy demand, with substantially reduced leakage, making it more efficient at containing volatile or hazardous fluids, aqueous solutions, and slurry suspensions. In addition, mechanical seals require no maintenance once installed.

A mechanical seal is comprised of a station­ary primary element fixed within the pump housing, and a rotating mating element fixed to the shaft. Precisely machined, these two com­ponents are pressed together by a flexible load element, meeting at a wear face, while the extreme tolerance precisions between the two elements minimize leakage. The wear faces are supported on an extremely thin lubricating film, typically 0.25 microns (9.8 micro inches) in thick­ness.

Available in a wide variety of types, arrange­ments, and materi­als, mechanical seals are found in most centrifu­gal pumps today. Advantages include the following:

Minimized water consumption and leakage—mechanical seals require very little flush water to be injected into the seal chamber. Compression packing used in abra­sive pumping applications requires significant water volumes to be injected into the stuffing box. A mechanical seal in the same service requires only a small fraction of this water volume.

Seals create an extremely restric­tive leak path perpendicular to the axis of the shaft and between the two sliding seal faces. This results in almost no leakage to the atmosphere.

Reduced power consumption—the amount of power required to drive a mechanical seal is as much as 80% less when compared to compression packing, primarily because the seal faces have less frictional energy losses due to the extremely precise mating between the stationary and rotating elements. Additional energy reduction requirements take the form of reduced need for flush water to be pumped into the seal.

Dual mechanical seals—designed to ensure maximum sealing safety, dual mechanical seals are typically defined as a single assembly that contains a pair of seals. A cavity is formed between the two seals within the assembly, which is filled with a bar­rier or buffer fluid that separates the pumped liquid from the atmosphere and environment.

Dual mechanical seals allow for near complete control over the seal operat­ing environment and the fluid film lubri­cating the seal faces. They provide max­imum elimination of the fluid leakage being handled in centrifugal pumps.

Reduced environmental impact

Efforts made toward improving sustain­ability in industrial processes, whether by reduced water and energy use, or by eliminating harmful fluid and gas discharge, reduce both environmental impact and operational costs.

Mechanical seals in centrifugal pumps, and particularly dual mechani­cal seals, are well-suited to reduce or eliminate volatile or hazardous fluids, and their harmful vapors, from escap­ing into the environment. They should be specified as the standard sealing solution, particularly when the pumped fluids present a safety, health, or envi­ronmental hazard. OG

Mark Savage is metal bellows engi­neering product group manager and Sam Ajram is global product marketing manager at John Crane Inc.

Original content can be found at Oil and Gas Engineering.

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