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Pump Shaft Sealing
Chapter 10
' Pump Shaft' Sea'ling Centrifugal and rotary positive displacement pumps have some sealing arrangements to keep pump fluid from leaking where the pump shaft penetrates the casing. These sealing arrangements may be packed glands or mechanical face seals. Seals also keep foreign material from entering the pump or lubricant from leaking out of bearings and transmissions. Reciprocating pumps have soft packings, molded elastomer rings, piston rings, and metal bushings. Most centrifugal and rotary pumps still have packings, but with mechanical seals becoming more reliable and economical, companies use mechanical seals even in water services. Packed Glands
Originally, a piece of cotton or hemp rope flushed by the pump fluid served as the pump packing. People in many parts of the world still use asbestos saturated with binders and lubricants, but the potential danger inherent in the material limits its use in the industrialized nations. Modern packings are: • • • • • • •
Braided graphite Expanded graphite in preshaped rings or in rolls Braided carbon fiber Polytetrafluoroethylene (PTFE) fiber Braided or PTFE impregnated with graphite Extruded PTFE mixed with graphite Braided aramide fiber coated with PTFE
A packing gland (Figure 10.1) works through compression. The rings are compressed with enough force to keep the pump fluid from leaking through the shaft. Even though the packing tings are either self-lubricated or grease lubricated, the pumped fluid adds to the lubrication and also cools the packing. Heat breaks down packings. To avoid heat buildup, the packing gland should allow some fluid to leak through. Too tight a gland will destroy the packing by heat deterioration. A lantern ring placed in the middle of the packing will add cooling fluid. Addition of a clean flushing fluid through the lantern ring is desirable when the pumped fluid is abrasive. Abrasive fluid or too tight a packing gland will eventually scour the part of the shaft beneath the packing. To prolong the shaft life, manufacturers install replaceable shaft sleeves where the shaft comes into contact with the packing. The advantages of packings over mechanical seals are that the packings save money initially and are easier to install and replace. 74
Pump Shaft Sealing FLUSHING
PACKING
75
FLUID
RING
Figure 10.1 Packed Gland Configuration
Mechanical Face Seals
These have been around since the beginning of the twentieth century. The first mechanical seals (Figure 10.2) simply consisted of a shaft collar turning directly against a machined part of the pump casing. Lack of suitable material and production techniques prevented the seals from being accepted by industry. Later the automobile industry began using a rubber V-ring based on the same principle as the original mechanical seal. With the development of synthetic elastomers and high-quality alloy steel, the chemical industry soon followed suit. Now mechanical face seals (Figure 10.3) are common in all industries. Plants turn them out by the millions, in small batches, or individually, to a buyer's specification. Processes handling liquids as diverse as sludge, acids, and hydrocarbons--with or without hydrogen sulfite, beer, plastic, and poisonous gas and liquids--use mechanical seals. In the 1940s, maximum seal pressure limits reached about 300 psig with shaft speeds not exceeding 35 ft/sec, but modem seals can tolerate more than 3,000 psig and speeds over 40,000 rpm.
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Practical Introduction to Pumping Technology • //
/
AXIAL
FORCE
P /Z)J
/} 0
-,q
Figure 10.2 Early Face Seal
General Design
Mechanical face seals may have either an axial force pressing a precision-lapped floating ring face against a fixed counterpart, or a fixed ring pressing against a floating ring. Springs or bellows press the faces together. O-rings, V-rings, U-cups, or other types keep the fluids from leaking along the surface of the shaft. Today almost all mechanical face seals are cartridge types, which means the seals arrive at the customers already assembled. Exceptions include overhung and very large pumps, whose weight may limit their use.
ROTATING
FACE S T A T I O N ~ ¥
-}
SPRING
Figure 10.3 Mechanical Face Seal
FACE
Pump Shaft Sealing
77
Mechanical seals for centrifugal and rotary positive displacement pumps break down into various categories. Internal Seal Arrangements
These arrangements (Figure 10.4) conduct the leakage radially inward. Both internal and external seals may have static or rotating floating tings. The great majority of centrifugal pumps using mechanical seals has an internal seal arrangement. External Seal Arrangements
These seals (Figure 10.5) conduct the seal leakage outward. The arrangement favors highly corrosive liquids and seals rotating at more than 4,000 rpm. Balanced and Unbalanced Seals
Balanced and unbalanced seals (Figures 10.6 and 10.7, respectively) differ according to the hydraulic loading of the seal faces. The ratio (A) of the hydraulic recess area (Ah) divided by the interface area (Ai), or A -- Ah/Ai, determines whether the seal is balanced or unbalanced. When A is smaller than 1, the seal is balanced, and when it exceeds 1, unbalanced. Mostly, the area ratio for unbalanced seals is 1.1 and 1.2, whereas the ratio for balanced seals may vary from 0.6 to 0.9. The risk of face seizing diminishes with lower area ratios, but the possibility of face separation increases.
/
Figure 10.4 Internal Seal Arrangement
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Practical Introduction to Pumping Technology
Figure 10.5 Extemal Seal Arrangement To determine when to use a balanced seal, apply the PV formula, which is the product of the sealing pressure in the stuffing box (P) and the peripheral velocity of the mechanical seal (V), measured at the mean diameter of the seal faces. The following formulas will determine P and V: P --- suction pressure + 0.25 x (discharge pressure - suction pressure)
For vertical turbine pumps, P equals the discharge pressure. Medium diameter (D m) - outside diameter (OD) + inside diameter (ID)/2
(diameters in inches) V --- D m x rpm/(3.82 ft/min) Use 1.5 times the shaft diameter for D m when you don't know this figure. As a rule of thumb, avoid using unbalanced seals for PVs less than 125,000 psig ft/min and for liquids with a specific gravity less than 0.7.
Basic Seal Types
Single Seals Single unbalanced or balanced seals are adequate for most services. The pumped liquid may wash the seal faces, or the flush fluid may provide an outside source of
Pump Shaft Sealing /
Figure 10.6 Balanced Mechanical Seal
Figure 10.7 Unbalanced Mechanical Seal
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Practical Introduction to Pumping Technology
clean liquid. Placing a bronze, close-clearance carbon or Teflon TM auxiliary bushing at the seal's atmospheric side gives added protection and provides a place for leak detection equipment. Except for water service, the radial clearances at the atmospheric side of the seal preferably should not measure less than 'A in. Some companies recommend an auxiliary gas seal for certain applications. Tandem Seals In this type (Figure 10.8), two single mechanical seals occur in series. The pumped liquid flushes the faces of the inner seal, whereas a clean outside buffer fluid flushes the secondary seal faces. The following conditions call for tandem seals:
Lack of adequate quench fluid, with carbon forming at the seal's atmospheric side or with the pumped liquid crystallizing at the same spot • Presence of toxic liquid that will vaporize at atmospheric conditions, causing fire hazards • A need to contain leakage from the primary seal and to delay leakage in case of a catastrophic failure of the primary seal • Higher stuffing-box pressures than a single mechanical seal can tolerate •
Double Mechanical Seals These seals (Figure 10.9) are placed face to face, with both faces flushed by the same buffer fluid from an outside source. It is desirable to design the seal at the product side so the product fluid cannot enter the seal if the buffer fluid is shutoff. The following conditions call for double mechanical seals:
The difference between the stuffing-box pressure and the pumped liquid's vapor pressure is less than 25 psi. • The pumped fluid is abrasive or highly corrosive, and there is no adequate flush liquid. •
Face Material
Metal seal faces are not practical because of the high friction coefficient, which creates the risk of seizure and overheating during emergency conditions. In hydrocarbon service, steel with a high graphite content runs against cast or sintered metals. However, carbon, plastic, and ceramic generally are run against metals, metal oxides, and carbides. If the material for the mating faces matches, one of the faces must be of a different grade of material. Mechanical seals are code classified, usually according to API Standard 610, though many manufacturers use their own codes. Appendix 8 shows suggested matedais for various liquids. For instance, an oil company may have the following specification for wet crude, pumped in several different conditions: • Use code BSAFJ. Flush primary seal with dry crude from an outside source. The auxiliary gas seal shall be BSPFX, where X shall be carbon against sapphire. • Use code BSTFM if wet crude flush is not available. Hush shall be clean water.
Pump Shaft Sealing
I
/ // ////
Figure 10.8 Tandem Mechanical Seal
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!
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Figure 10.9 Double Mechanical Seal
R \ Tx\ ~\
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Practical Introduction to Pumping Technology
Use BDPFX if neither dry crude nor clean fresh water is available. X in the seal next to the product shall be tungsten carbide against tungsten carbide (TC vs. TC). X in the outer seal shall be carbon against silicon carbide (C vs. SC).
Cyclone Separator When flushing a mechanical seal with pumped fluid that contains abrasives or other contaminants, a hydrocyclone separator installed in the flush line shall clean the fluid. Do not use hydrocyclones when the pressure differential in the unit is less than 25 psi.
Flush and Quench Fluids You may choose to arrange flushing and quenching piping according to API Standard 610.
Stuffing-Box Cooling Cooling stuffing boxes is essential for pumps handling liquids at 212°F or higher. To do this, either run water through a jacket around the bearing housing or recirculate the flushing fluid through a heat exchanger.
Buffer Fluid Schemes Tandem and double mechanical seals need buffer fluid, which may be any clean fluid, preferably water. The design pressure for the vessel holding a tandem buffer fluid shall be strong enough to withstand maximum stuffing-box pressure. The system for a double mechanical seal buffer fluid shall be able to withstand at least 50 psig more than the pump's suction pressure.
Face Seal Life Expectancy Literature by seal manufacturers claim mechanical seals operating on design conditions may last up to 100,000 hr. This equates with at least ten years' service, which may be overly optimistic. Seals rarely run for ten years under design conditions. You can rarely avoid changes in process, liquid characteristic, or other factors. A more realistic time frame may be from two to five years.
' Pump Shaft' Sea'ling Centrifugal and rotary positive displacement pumps have some sealing arrangements to keep pump fluid from leaking where the pump shaft penetrates the casing. These sealing arrangements may be packed glands or mechanical face seals. Seals also keep foreign material from entering the pump or lubricant from leaking out of bearings and transmissions. Reciprocating pumps have soft packings, molded elastomer rings, piston rings, and metal bushings. Most centrifugal and rotary pumps still have packings, but with mechanical seals becoming more reliable and economical, companies use mechanical seals even in water services. Packed Glands
Originally, a piece of cotton or hemp rope flushed by the pump fluid served as the pump packing. People in many parts of the world still use asbestos saturated with binders and lubricants, but the potential danger inherent in the material limits its use in the industrialized nations. Modern packings are: • • • • • • •
Braided graphite Expanded graphite in preshaped rings or in rolls Braided carbon fiber Polytetrafluoroethylene (PTFE) fiber Braided or PTFE impregnated with graphite Extruded PTFE mixed with graphite Braided aramide fiber coated with PTFE
A packing gland (Figure 10.1) works through compression. The rings are compressed with enough force to keep the pump fluid from leaking through the shaft. Even though the packing tings are either self-lubricated or grease lubricated, the pumped fluid adds to the lubrication and also cools the packing. Heat breaks down packings. To avoid heat buildup, the packing gland should allow some fluid to leak through. Too tight a gland will destroy the packing by heat deterioration. A lantern ring placed in the middle of the packing will add cooling fluid. Addition of a clean flushing fluid through the lantern ring is desirable when the pumped fluid is abrasive. Abrasive fluid or too tight a packing gland will eventually scour the part of the shaft beneath the packing. To prolong the shaft life, manufacturers install replaceable shaft sleeves where the shaft comes into contact with the packing. The advantages of packings over mechanical seals are that the packings save money initially and are easier to install and replace. 74
Pump Shaft Sealing FLUSHING
PACKING
75
FLUID
RING
Figure 10.1 Packed Gland Configuration
Mechanical Face Seals
These have been around since the beginning of the twentieth century. The first mechanical seals (Figure 10.2) simply consisted of a shaft collar turning directly against a machined part of the pump casing. Lack of suitable material and production techniques prevented the seals from being accepted by industry. Later the automobile industry began using a rubber V-ring based on the same principle as the original mechanical seal. With the development of synthetic elastomers and high-quality alloy steel, the chemical industry soon followed suit. Now mechanical face seals (Figure 10.3) are common in all industries. Plants turn them out by the millions, in small batches, or individually, to a buyer's specification. Processes handling liquids as diverse as sludge, acids, and hydrocarbons--with or without hydrogen sulfite, beer, plastic, and poisonous gas and liquids--use mechanical seals. In the 1940s, maximum seal pressure limits reached about 300 psig with shaft speeds not exceeding 35 ft/sec, but modem seals can tolerate more than 3,000 psig and speeds over 40,000 rpm.
76
Practical Introduction to Pumping Technology • //
/
AXIAL
FORCE
P /Z)J
/} 0
-,q
Figure 10.2 Early Face Seal
General Design
Mechanical face seals may have either an axial force pressing a precision-lapped floating ring face against a fixed counterpart, or a fixed ring pressing against a floating ring. Springs or bellows press the faces together. O-rings, V-rings, U-cups, or other types keep the fluids from leaking along the surface of the shaft. Today almost all mechanical face seals are cartridge types, which means the seals arrive at the customers already assembled. Exceptions include overhung and very large pumps, whose weight may limit their use.
ROTATING
FACE S T A T I O N ~ ¥
-}
SPRING
Figure 10.3 Mechanical Face Seal
FACE
Pump Shaft Sealing
77
Mechanical seals for centrifugal and rotary positive displacement pumps break down into various categories. Internal Seal Arrangements
These arrangements (Figure 10.4) conduct the leakage radially inward. Both internal and external seals may have static or rotating floating tings. The great majority of centrifugal pumps using mechanical seals has an internal seal arrangement. External Seal Arrangements
These seals (Figure 10.5) conduct the seal leakage outward. The arrangement favors highly corrosive liquids and seals rotating at more than 4,000 rpm. Balanced and Unbalanced Seals
Balanced and unbalanced seals (Figures 10.6 and 10.7, respectively) differ according to the hydraulic loading of the seal faces. The ratio (A) of the hydraulic recess area (Ah) divided by the interface area (Ai), or A -- Ah/Ai, determines whether the seal is balanced or unbalanced. When A is smaller than 1, the seal is balanced, and when it exceeds 1, unbalanced. Mostly, the area ratio for unbalanced seals is 1.1 and 1.2, whereas the ratio for balanced seals may vary from 0.6 to 0.9. The risk of face seizing diminishes with lower area ratios, but the possibility of face separation increases.
/
Figure 10.4 Internal Seal Arrangement
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Practical Introduction to Pumping Technology
Figure 10.5 Extemal Seal Arrangement To determine when to use a balanced seal, apply the PV formula, which is the product of the sealing pressure in the stuffing box (P) and the peripheral velocity of the mechanical seal (V), measured at the mean diameter of the seal faces. The following formulas will determine P and V: P --- suction pressure + 0.25 x (discharge pressure - suction pressure)
For vertical turbine pumps, P equals the discharge pressure. Medium diameter (D m) - outside diameter (OD) + inside diameter (ID)/2
(diameters in inches) V --- D m x rpm/(3.82 ft/min) Use 1.5 times the shaft diameter for D m when you don't know this figure. As a rule of thumb, avoid using unbalanced seals for PVs less than 125,000 psig ft/min and for liquids with a specific gravity less than 0.7.
Basic Seal Types
Single Seals Single unbalanced or balanced seals are adequate for most services. The pumped liquid may wash the seal faces, or the flush fluid may provide an outside source of
Pump Shaft Sealing /
Figure 10.6 Balanced Mechanical Seal
Figure 10.7 Unbalanced Mechanical Seal
79
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Practical Introduction to Pumping Technology
clean liquid. Placing a bronze, close-clearance carbon or Teflon TM auxiliary bushing at the seal's atmospheric side gives added protection and provides a place for leak detection equipment. Except for water service, the radial clearances at the atmospheric side of the seal preferably should not measure less than 'A in. Some companies recommend an auxiliary gas seal for certain applications. Tandem Seals In this type (Figure 10.8), two single mechanical seals occur in series. The pumped liquid flushes the faces of the inner seal, whereas a clean outside buffer fluid flushes the secondary seal faces. The following conditions call for tandem seals:
Lack of adequate quench fluid, with carbon forming at the seal's atmospheric side or with the pumped liquid crystallizing at the same spot • Presence of toxic liquid that will vaporize at atmospheric conditions, causing fire hazards • A need to contain leakage from the primary seal and to delay leakage in case of a catastrophic failure of the primary seal • Higher stuffing-box pressures than a single mechanical seal can tolerate •
Double Mechanical Seals These seals (Figure 10.9) are placed face to face, with both faces flushed by the same buffer fluid from an outside source. It is desirable to design the seal at the product side so the product fluid cannot enter the seal if the buffer fluid is shutoff. The following conditions call for double mechanical seals:
The difference between the stuffing-box pressure and the pumped liquid's vapor pressure is less than 25 psi. • The pumped fluid is abrasive or highly corrosive, and there is no adequate flush liquid. •
Face Material
Metal seal faces are not practical because of the high friction coefficient, which creates the risk of seizure and overheating during emergency conditions. In hydrocarbon service, steel with a high graphite content runs against cast or sintered metals. However, carbon, plastic, and ceramic generally are run against metals, metal oxides, and carbides. If the material for the mating faces matches, one of the faces must be of a different grade of material. Mechanical seals are code classified, usually according to API Standard 610, though many manufacturers use their own codes. Appendix 8 shows suggested matedais for various liquids. For instance, an oil company may have the following specification for wet crude, pumped in several different conditions: • Use code BSAFJ. Flush primary seal with dry crude from an outside source. The auxiliary gas seal shall be BSPFX, where X shall be carbon against sapphire. • Use code BSTFM if wet crude flush is not available. Hush shall be clean water.
Pump Shaft Sealing
I
/ // ////
Figure 10.8 Tandem Mechanical Seal
/
-
!
~"
Figure 10.9 Double Mechanical Seal
R \ Tx\ ~\
81
82 •
Practical Introduction to Pumping Technology
Use BDPFX if neither dry crude nor clean fresh water is available. X in the seal next to the product shall be tungsten carbide against tungsten carbide (TC vs. TC). X in the outer seal shall be carbon against silicon carbide (C vs. SC).
Cyclone Separator When flushing a mechanical seal with pumped fluid that contains abrasives or other contaminants, a hydrocyclone separator installed in the flush line shall clean the fluid. Do not use hydrocyclones when the pressure differential in the unit is less than 25 psi.
Flush and Quench Fluids You may choose to arrange flushing and quenching piping according to API Standard 610.
Stuffing-Box Cooling Cooling stuffing boxes is essential for pumps handling liquids at 212°F or higher. To do this, either run water through a jacket around the bearing housing or recirculate the flushing fluid through a heat exchanger.
Buffer Fluid Schemes Tandem and double mechanical seals need buffer fluid, which may be any clean fluid, preferably water. The design pressure for the vessel holding a tandem buffer fluid shall be strong enough to withstand maximum stuffing-box pressure. The system for a double mechanical seal buffer fluid shall be able to withstand at least 50 psig more than the pump's suction pressure.
Face Seal Life Expectancy Literature by seal manufacturers claim mechanical seals operating on design conditions may last up to 100,000 hr. This equates with at least ten years' service, which may be overly optimistic. Seals rarely run for ten years under design conditions. You can rarely avoid changes in process, liquid characteristic, or other factors. A more realistic time frame may be from two to five years.