Seal Selection and
Application - Selecting the Best Seal for Specific
Applications
Controlling the fluid inside a process is usually of great concern to engineers and
technicians who design and work on the process. There are many issues that must be
contended with: calculation of mechanical and fluid efficiencies, mechanical loads,
vibrations, thermal and pressure limits and controls, etc. These items consume a great
deal of time and attention.
Equally disastrous, leakage problems must be considered in
all fluid flow applications. Often, an incorrect seal decision can result in many hours
spent pursuing a better seal for some leaking apparatus. There are many reasons why
experienced process engineers are demanding about the sealing techniques used on equipment
in their factories, including concerns about wasted product (profits,) maintenance costs,
safety (liability,) and environmental concerns. These reasons combine into one important
point: a leaking machine is a failing machine.
The selection of the seals for pump application is one of the most difficult tasks in
applying the pump to the project. Seal selection is always a key decision when reviewing
an application. Because of the sensitivity of the plant employees to the leakage issue,
the ultimate success or failure of the pump is decided by the success of the seal. Indeed,
the pump might be ideally applied, as far as the pumping mechanism is concerned, producing
ideal flow conditions, but if it drips on the floor more than expected, it is a failure.
Thus, the success of a $10,000 pump might rest on the successful application and
installation of a $1 o-ring seal. Seal selection cannot be trivialized.
Detailed application data and information about the product are critical, including
information about the consistency of the product over the operating ranges of temperature,
speed and pressure. Small variations in fluid chemistry, abrasive content, temperature or
pressure can all lead to premature seal failure. Careful questioning can help lead to
correct application. For example, lubricating ability of the fluid is important to know,
yet difficult to quantify.
This information is sometimes difficult to obtain for other
reasons. Some customers refuse to divulge detailed information about the product or
process because they consider this proprietary. In these cases the customer must be
encouraged into providing seal material recommendations and accurate information about
abrasive content.
Separating seals into categories so they can be discussed can be complicated. One way to
separate them is into Static seal and Dynamic seal groups. There are many sub-groups of
seals, and there is some overlap of these two categories, but we will try to deal with
these seals in a straightforward, logical way and recommend how they can be applied into
specific applications.
Static seals are those which function between
surfaces which do not move relative to one another, such as between a pump body and a seal
housing.
Dynamic seals are those which prevent leakage
between two surfaces which are moving relative to each other, such as a shaft turning
inside a housing.
Selection of both static and dynamic seals is a critical part
of the equation for applying a pump. Both types of seals must function effectively in
order for the product to be a success. Most seal failures in rotating equipment occur with
the dynamic seal at the driving shaft, but leakage at the pump housing still can occur if
the static seal is improperly applied. Care must be taken to ensure that all the sealing
is effective throughout the operating range of the equipment, considering temperature and
pressure changes.
Static Seals
Static sealing elements, as their classification implies, remain stationary relative to
the surfaces they are sealing against. An example of a static sealing element would be a
gasket sandwiched between two plates of a flange assembly. In this example, the sealing
efficiency of the assembly is improved by the conforming/compliant nature of the gasket
layer, thus preventing a leak which would have occurred if the two flange faces were
pressed together without a gasket between. This flange example is called a face seal
application, because the static seal is between two faces of the objects to be sealed.
In Zenith pumps, static seals prevent leakage from occurring
between the pump housing plates or between the seal housing and the pump body, or between
the pump and porting surface or fitting. These are face seal applications. Sealing between
the pump shaft and a device that turns with the shaft would also be a static sealing
application. Even though the drive shaft and device are moving relative to the pump body,
there is no motion between the device and the shaft.
There are various types of static seals which can be used in
Zenith pumps. Those most commonly found are metallic and non-metallic gaskets, elastomeric
and plastic o-rings and lip seals, packings, metal sealing rings and metal-to-metal seals.
Metal-to-Metal 
Metal-to-metal seals are the most common static seals in Zenith pumps. The metal-to-metal
seal has the greatest range of temperature and pressure of any static seal.
This type seal relies on two metal surfaces being brough
together and clamped so that any gap remaining between them is so small there is no
leakage. This seal is based on the idea that if this gap is extremely small, the fluid
pressure inside the pump cannot force the fluid out through the small gap due to head
losses.
As fluid viscosity goes up, metal-to-metal seals become more
capable (higher leakage pressure,) and work extremely well for fluids with viscosity
greater than 100 centipoise (cps). For applications where the fluid viscosity is lower
than 100 cps, metal-to-metal seals can still perform well, but the likelihood of a leak is
increased as the viscosity goes down and pressure goes up. It is best to enhance a
metal-to-metal static seal by adding a more compliant seal if the fluid viscosity is below
100 cps and fluid leak is dangerous because of toxicity, flammability, etc. Metal-to-metal
seals should be enhanced with a more compliant seal for applications where the fluid
viscosity is below 5 cps and the pressure is above 20 psi.
For applications where the inlet pressure falls below 1
atmosphere (vacuum), the pump with metal-to-metal seals may allow air to be pulled in. If
this is not acceptable for the application, a pump design should be chosen which includes
a better static seal using an elastomeric sealing element.
Metal-to-metal seals are used between the pump plates,
between the plates and the seal housing and between the pump and its mounting surface, in
some installations. Dryseal (NPT, BSP) pipe threads which are not enhanced with PTFE tape
are also metal-to-metal seals.
Gaskets
Gaskets are usually thin, compliant structures which can be compressed between
two plates. They will conform to the irregularities of the plate surfaces, providing a
positive static seal. The compression loading of the gasket prevents it from being
extruded through the space between the plates. Gaskets are rarely used in Zenith pump
assemblies, since they are compressed, and pump designs usually require careful control of
the accumulated thickness of the assembly in order to perform well. Also, structurally,
the gasket layer causes the compression stiffness of the bolted, pump assembly to be
dramatically reduced, which is undesirable. This structural change would dramatically
reduce the ability to control the preload of the fasteners, and is one of the greatest
reasons that gaskets are not used between the pump plates.
Gaskets of paper, PTFE or metal can be used on Zenith pump
installations between the pump body and the seal housing or between the pump and the
mounting surface. This is the most common application of gaskets to Zenith pumps.
Metal gaskets are sometimes used with good result at the
mounting surface of a rear-ported design to help improve the seal. In these installations,
the gasket is chosen from a softer metal with a compression stiffness less than the pump
plate materials, and is intended to offer slightly compliant layer which would help seal
against a slightly irregular surface (Ra 32, etc.). Corrugated gasket designs are used,
also, to improve the compliant nature of the gasket.
Elastomeric O-rings
Elastomeric o-rings provide excellent static sealing if properly applied to the fluid and
pump. O-rings fit into grooves, and so are controlled, by the groove depth, to prevent
over- or under-compression. Because the o-ring is contained in a groove, it does not
affect the compressive stiffness of the pump body assembly.
O-rings will provide adequate sealing of thin fluids
(<10cps) up to high pressures without special consideration. They will also work well
to prevent air or gas from being pulled into the pump during operation. In face seal
applications and with a properly designed and manufactured o-ring groove, the pressure
limit on the joint is quite high, since the extrusion gap is near zero and o-ring is
backed properly by the groove itself.
O-rings are a common type of static sealing device on Zenith
pumps since they do not affect the overall stack height of the pump plates (no change to
gear clearances). The o-ring type seal can handle a large temperature range if the seal is
chosen well. Fluorocarbon (Viton®, etc.) seals are good for applications to 450° F, and
Kalrez® seals can withstand applications to 650° F.
PTFE (Teflon®, etc.) encapsulated o-rings can be used in
applications requiring the chemical resistance of PTFE, but also requiring the compliant
nature of an elastomer. These o-rings are assemblies, where a PTFE jacket is cast around a
standard elastomer o-ring of Fluorocarbon or EPDM (EPR). The elastomer o-ring acts like a
spring, and is not contacted by the chemical.
Solid, PTFE o-rings are sometimes used in Zenith pumps, but
usually in customer required applications only. These seals take a compression-set,
meaning that when they are compressed, they permanently deform to take on the new shape
and no longer apply sealing force the groove and sealing plate. PTFE encapsulated o-rings
are superior for most applications.
Operating temperature ranges for these seals is that of the
elastomer inside, not for the PTFE jacket, so a seal with a Fluorocarbon elastomer still
is limited to 450° F. The compact size of the o-ring groove lends itself well to Zenith
pump designs. These are the second most common types of static seal in our pumps.
Lip Seals
Elastomer or PTFE Lip Seals can be used as static seals, similarly to the way o-rings are
used, with the seal placed in a groove in the housing or plate face. The same advantage
applies for the compressive stiffness. Widely used as dynamic seals, these sealing
elements are rarely used as static seals in Zenith pumps.
Elastomeric lip seals are as compliant as o-rings and offer
improve efficiency for chemicals in the fractional 10 cps viscosity range. Pressure
sealing capability can reach to 10,000 psi for certain applications, even with thin
fluids. The size of the sealing element prevents its use in many applications, where the
compact size of the elastomeric o-ring is more attractive.
PTFE lip seals are compliant and resilient, due to the metal
or elastomeric spring which energizes the lips, similar to the PTFE encapsulates o-ring
seals. This type of seal has a large operating range of temperature and pressure, the
second largest of the static seals (metal-to-metal seal is the largest). This type of lip
seal is used where PTFE is required, where the temperature range will exceed 450° F,
where the sealing pressure is greater than that which other seals will handle, or where
the customer requires this type of seal.
Packings
Packings can be used as static seals, such as in valve stems, but are rarely used in
Zenith pumps. Fluid compatibility issues would dictate this use, perhaps for extreme
temperature service, where compressed graphite might be the only adequate sealing
material.
Metal Rings
Metal seal rings of varying cross-section geometry are used successfully in place of
Elastomeric o-rings or Lip Seals for static service. These seals are used where it is not
possible to finish the joint properly for metal-to-metal seal and/or temperature limits
prevent o-rings or Lip Seals from being used. Metal seal rings are usually made from, or
plated with, a softer, compliant metal like nickel or silver, which will conform to the
surface irregularities of the housing. These rings seal well against moderately smooth
surfaces of 16 to 32 Ra.
Metal o-rings, C-rings and V-rings, etc. are used in Zenith
pumps with good success, mostly in higher temperature applications (>450° F,) and with
more viscous fluids (>100 poise).
For all of the following sets of conditions, it is assumed that the sealing surfaces are
clean and not damaged. Dirt and contamination can cause a static seal to fail, regardless
of seal design and operating conditions. Scratches, nicks and dents can also cause failure
to seal.
For applications under 10 cps or where the inlet pressure
becomes a vacuum, leakage between the pump plates or between the pump and the seal
housing is most likely due to inadequate static sealing compliance, which results in poor
sealing efficiency.
For applications between 10 cps and 10,000 cps (100
poise), leakage between the pump plates or between the pump and the seal housing could
be due to either inadequate static sealing compliance or inadequate clamping pressure. A
pressure calculation analysis will identify if plate deflection is likely to be causing
the problem, of if the static sealing method is at fault. Very small surface defects
(scratches, dents, nicks, corrosion, pitting, etc.) can cause the pump to leak. Care in
handling is extremely important for ensuring a leak free assembly.
For applications over 10,000 cps (100 poise), leakage
for a metal-to-metal seal between the pump plates or between the pump and the seal housing
are due to inadequate clamping pressure. The pump design needs to be stiffened, or
additional bolts need to be added to the assembly to prevent plate separation. Leakage for
other types of seals probably indicates the seal is beyond its pressure limit or the seal
is damaged, prior to installation.
Metering
System Design Requirements
Volumetric
Efficiency of Gear Pumps |
