Gasket Thickness Tolerance Knowledge Can Prevent Pump Leaks

The phrase “It’s just a small leak” is rarely found in the vocabulary of any maintenance manager, pump operator or pipefitter. Facing growing concerns such as tighter Environmental Protection Agency (EPA) emissions legislation, environmental issues, costly downtime and overall plant safety, plant personnel understand that overlooking a seemingly small issue—including a leaking pump casing—is a serious risk. To reduce these risks, pump users should carefully consider the characteristics of their equipment to ensure effective sealing and reduce leaks.

Seating Surface

The finish or the condition of the gasket seating surface has a definite effect on the ability of the gasket to establish a seal. Compressed non-asbestos gasket materials are porous and typically require a minimum gasket seating stress of 4,800 pounds per square inch (psi) to reduce or eliminate the porosity and achieve a proper seal. However, using a polytetrafluoroethylene (PTFE) gasket material (particularly glass-filled materials that reduce creep) eliminates this porosity issue. Regardless of which material is selected, whether non-asbestos or filled PTFE, sheet gasketing material is designed to have a seating stress applied that allows the gasket material to flow into the serrations and irregularities of the flange face. This “bite” of the serrations into the gasket material helps the gasket to resist the effects of internal pressure, creep and cold flow. Gasket creep is a major cause of gasket sealing issues and generally increases as the material thickness increases. Smooth finishes are usually found on pump casing gaskets and other machinery or flanged joints (except pipe flanges). When working with a smooth finish, consider using a thinner gasket to minimize the effects of creep and cold flow. Note that a thinner gasket and a smooth finish will require a higher compressive force (bolt torque) to establish a seal.

Pump Styles & Gasket Performance

While many pump types exist, axial split case and radial split case pumps are common in the industrial world. Both styles of pumps have pros and cons, specifically in terms of application gasket design. Proper sealing in axial split case pumps can be particularly troublesome, because the gaskets are mostly unconstrained and have an asymmetrical shape that can cause both high and low stress concentrations on the gasket (see Figure 1). Leaks and gasket blowouts occur most frequently in these low stress concentration areas. Radial split case pumps have fewer sealing issues because they have a more proportioned bolting pattern and enclosed gasket design. However, these pumps have smaller bolting areas, so the maximum amount of seating stress that must be applied to the gasket can make sealing difficult. In some cases, the seating stress cannot be increased because of the design, so altering the effecting sealing area of the gasket may be a feasible option.

Gasket Load Curves

Another effective tool for selecting a gasket material is a load and compression curve, which shows gasket stress being applied to the gasket versus the deflection while under that stress (see Figure 2). Compression curves help show how a gasket material densifies under increasing stress as the original porosity is reduced and closed. The material can withstand an increasing load until the stress reaches a maximum value at failure although compressing the gasket to this value would not be ideal or recommended. Compression curves are typically run on a loading and unloading cycle to determine the effects of leakage as gasket stress is reduced after initially being loaded to a higher value. Depending on how many data points are plotted, the results can be non-linear or linear. Interpretation of loading curves and other real-time data from the pump user is best left to the manufacturer’s applications engineer.

The Effects of Manufacturing Methods

Specific gasket sheet manufacturing methods can yield various degrees of tolerance within PTFE sheeting material. The gasket industry uses two main methods: the HS-10 calendar method and the skived method. With the calendar method, the sheet thickness variation within thinner gauge material, such as 1/64-inch and 1/32-inch of sheet material, is much more difficult to control than with skiving sheets. Calendared sheets are made between two rolls, one hot and one cold (see Image 1). The solvent-based mixture is added to the calendar, which pinches the mixture between these rolls. A problem with thickness tolerance can occur because both rolls have an outer machined crown. This allows the rolls to flatten out by the deflection caused with increasing nip (the pinching space between the two rolls) pressure from the added material. The operator is always competing against the deflection within the crown, so this method is operator-dependent. The method for making skived PTFE is similar to slicing veneer on a lathe. A hardened steel blade is mounted to the nose bar of the lathe, which is reinforced throughout the entire length of the blade (see Images 2 and 3). This makes the knife edge much more resistant to deflection, resulting in truer gauges with less variation in thickness throughout the sheet. Both manufacturing methods produce 1/64-inch and 1/32-inch thick PTFE materials that are ideal for pump casings. Typical gasket thickness range for skived 1/64-inch material is 0.014-0.021 inch with a +/- tolerance of 0.003 inch across the width of the sheet. The calendar method produces a typical gasket thickness for 1/64-inch material ranging from 0.014-0.021 inch with a +/- tolerance of 0.005-0.002 inch (based on available industry literature). Based on these numbers, the skived method provides more consistent gauge thicknesses, which can be a critical factor in sealing smooth-faced pump casings. References http://www.centrifugalpump.org/pump_axial_radial.html