I/EC G u i d e b o o k for Technical Management
EQUIPMENT AND DESIGN Without prior experience, this practical approach w i l l help to solve problems of . . . .
Mechanical Seals for Slurry Service by Robert Fornesi, Goulds Pumps, Inc.
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SLURRY is a "mixture of a liquid
and undissolved solids," either amorphous or crystalline. T h e amorphous-type solid usually found with organic chemicals is often not abrasive and can be sealed with a single seal. However, such a slurry has a tendency to gum up seal springs, resulting in possible seal failure. A single seal mounted in a n outside position is often a solution to this problem. In an outside position the pumpage is not in contact with the springs, so that their action is unaffected by the slurry. Abrasive solids are usually of the crystalline type, with size and degree of abrasiveness varying greatly. Solution to the problem of handling such abrasives is difficult because of factors other than particle size. O b viously, the smaller the particle, the easier it will get in between the faces. In addition, the amount of pressure exerted against the abrasives will reflect on how easily they get in between the mating surfaces. Finally, the liquid in which abrasive particles are held can determine whether particles will get in between faces. Because film thickness between the faces is dependent on the viscosity and surface tension of the given film, a heavy oil, for example, will make
a heavy film between the faces and allow comparatively larger particles to ride in its wake. With a comparatively low specific gravity hydrocarbon, a film of minimum thickness exists between the faces, and prevents entrance of particles of a size that would normally come through with oil. For these major reasons, any recommendation for the use of a single seal on an abrasive slurry would be a theoretical correlation of viscosity, pressure, surface tension, and particle size. Because this is difficult, previous experience is the most practical approach and, if unavailable, it is suggested that the following recommendations, listed in order of preference, be followed. In any of the three recommendations, seal materials should be the most suitable available to resist abrasion. Ceramic stationary seats are exceptionally hard and should be used whenever possible. In some cases, a plain single seal will give sufficient seal life to justify its use, even though a more expensive system lasts longer. Single S e a l , Clear Liquid Injection
When the single seal is used, the clear liquid is injected at a pressure slightly in excess of the pressure on
k Single
the stuffing box, thus isolating the mechanical seal from the abrasive pumpage and promoting flow into the pump. The main objection to this method is dilution of the product. To limit this dilution a restricting bushing is placed on the bottom of the box with a close clearance between it and the shaft. With a n accurate pressure regulation of the clear liquid, the dilution should be negligible. The clear liquid pressure should be maintained in excess of the box pressure even under nonoperating conditions, to prevent any entrance of abrasive pumpage to the seal area. The restricting bushing is normally carbon, although glassfilled Teflon is available if required. An alternate arrangement utilizes a V ring (usually Teflon) restriction in conjunction with a single outside seal. A stationary spring compressed in the box holds the V ring in position at the bottom. Clear liquid is injected into the box at slightly above pump pressure, thus promoting clear liquid flow into the pump, lubricating the V ring, and presenting entrance of pumpage into the box. The position of the V ring lip toward the pump prevents sudden pressure surges in the system from forcing pumpage into the seal cham-
seal with throttle bushing a n d clear liquid injection
•^ Single outside mounted seal f o r slurry o f amorphous-type
solid VOL. 5 1 , N O . 11
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NOVEMBER 1959
59A
I/EC
G u i d e b o o k for Technical M a n a g e m e n t — E Q U I P M E N T
• A.
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ber. This arrangement is less sensitive to pressure differences than the restricting bushing and therefore provides better control of product dilution. Clear liquid injection pressure is limited to the seal design, usually 35 p.s.i. for outside unbalanced seals or 150 p.s.i. for balanced seals. D o u b l e Seals
The double-type seal is the combination of two single seals within one stuffing box. This provides two sealing faces, one at the inboard end of the stuffing box and the other at the gland or outboard end. This arrangement excludes the pumpage from the stuffing box. An outside source of liquid to lubricate the seal faces from within the seal cavity is required. The circulating liquid should be at a pressure approximately 20 pounds greater than the maximum pressure, cither static or operating, that may occur at the inboard seal. This will ensure a continual contact of the seal faces and eliminate possible popping of the inner seal faces due to minor pressure surges within the pump system, ft will also provide a supply of clear liquid between the seal faces for proper lubrication. If system is pressurized during nonoperating periods, circulating pressure should be maintained also. The main concern involving double-type mechanical seals is the special pressure unit often required to pressurize the stuffing boxes. For proper trouble-free operation, good maintenance is essential. 60 A
DESIGN
Single seal with V-ring restriction and clear liquid injection Pipe tap connection for clear liquid injection
Double seal as installed in Goulds Fig. 3 7 1 5 stainless steel pump
With double mechanical seals there is possible contamination of the pumpage by the circulating liquid. Leakage could occur between the seal chamber and the pumpage, contaminate the fluid being handled, and render it unsuitable for future use in the process. A compatible circulating liquid should be selected. In applications involving slurries, a clear solution of the slurry liquor or water, if it is a water-base slurry, should be used. Double mechanical seal parts are in constant immersion of the circulating liquid rather than the pumpage. Seal materials should be selected to withstand the corrosive action of the circulating liquid as well as the pumpage. Quench Gland
The single seal with a quench style gland is suitable for use only with liquids or slurries where abrasive conditions are caused by contact of the liquid with the atmosphere. This pumpage may be a saturated salt solution where the solution cools upon contact with the atmosphere, to cause crystallization of some salts at the seal faces. By circulating a quench liquid around the seal faces, any salts formed will be dissolved in and flushed away by the quench liquid. While a cool quench liquid may dissolve salts on the exterior of the faces, a quench liquid at a temperature near the pumping temperature is recommended. This will prevent cooling of the liquid film between the faces, thus preventing crystallization of salts in this critical area.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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The pumpage may be a liquid such as sodium silicate, which hardens upon contact with the atmosphere. In this instance, the quench liquid retained by the quench gland acts as a buffer zone between the atmosphere and the pumpage at the seal faces. A quench gland has two tapped holes which allow for flushing of a clear liquid around the outside of the seal faces, thus dissolving salts or crystals present. The quench liquid cannot contaminate the pumpage and is from an outside source. It is retained in the gland by a closefitting bushing of bronze or carbon. If required, two rings of packing and an auxiliary gland can be supplied in place of the close-fitting bushing. Quench lines should be flexible, with valves on both sides of the gland. These valves should be controlled to allow sufficient liquid to fill the quench gland recess and flow out through the line. The pressure should be kept low enough (a few pounds) to prevent spraying of quench liquid out between the close fitting bushing and the shaft or sleeve. Quench liquid should not be turned off, even though pump is not operating, since vaporization or crystallization may continue.
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