Characteristics and design diagrams of pump shaft seals, their reliability and leak-tightness indices, andcomparative analysis of cost of leak-tightening are given.

Leak tightening of shafts is a serious problem of sealing technology in pump making. Correct choice of seals is ofgreat importance for reliable operation of pumps because as much as 70% of their failures occur due to failure of the seals,the cost of which may be as much as 20% of the pump price.

Characteristics of Pump Shaft Seals. Nowadays, pumps are fitted essentially with mechanical face (FS) and radi-al gland (GS) seals. Furthermore, in the last 15 years face gland seals (FGS) have been in use in Russia [1]. The choice ofseal type depends on the operation conditions, equipment specifications, and economic feasibility [2]. The criteria for eval-uation of seals are maximum safety and minimum damage to ecology. But the optimum version of the seal can be chosen ineach specific case only by serious analysis.

The reliability of seals depends on their service life and the probability of sudden failure. The service life is the peri-od of seal service until its replacement with dismantling of the pump. In this regard, depending on the seal type and condi-tions of operation, the nonfailure operating time (mean-time-between-failures) of the seal, i.e., the time until attainment ofimpermissible leakage of the seal, and the service (operating) life of the seal are not the same. Let us examine from this stand-point the types of seals shown in Fig. 1.

GS (gland seal) is a controllable assembly (tightening of the pressing mechanism without dismantling the pump pre-vents elevation of the leakage level above the permissible limit). Since the increase in leakage depends on relatively slowwear of the friction pair, the possibility of sudden failure is not large, i.e., if the operation is proper, sudden troubling leak-age does not occur. For replacement of gland packing, disassembling of the pump is not required. The service life of theassembly is determined by the attainment of maximally permissible wear of the protective bushing of the shaft, forthe replacement of which dismounting and disassembling of the pump are required.

Thus, GS is a controllable assembly and can be restored (repaired) periodically in the course of its use.FS (face seal) is an uncontrollable assembly and is not easily restorable (repairable) under the operation conditions:

disassembling of the pump and replacement of the seal are required. The service life of FS with a ground friction pair isunequivocally determined by the nonfailure operating time until attainment of impermissible leakage. Furthermore, in mostcases, failure of the FS occurs suddenly and determines its service life. The time of nonfailure operation of a specific FSis impossible to predict because the reason for the failures is either loss of mobility or failure of one of the sealing elements.

Attempts to use detachable friction pair [4] did not gain popularity because of complicated design and difficulty inassembling. The major lines of FS development are improving the materials of the friction pair and reducing the cost foruncomplicated conditions.

FGS (face gland seal) is a restorable (repairable) assembly. The leak-tightness of FGS is restored by replacing oneof the components of the face friction pair (ring of gland packing). This operation does not require complete disassembling

Chemical and Petroleum Engineering, Vol. 42, Nos. 910, 2006

CRITERIA FOR SELECTING PUMP SHAFT SEALS

Ya. Z. Gaft

Izogerm OOO, Moscow. Translated from Khimicheskoe i Neftegazovoe Mashinostroenie, No.10, pp. 2629,October, 2006.

0009-2355/06/0910-0576 2006 Springer Science+Business Media, Inc.576

of the pump and the service life is determined by the wear of the hard thrust journal, for the replacement of which disas-sembling of the pump is required.

Let us consider the efficiency of various types of seals and the criteria for their selection for the pumps currently inoperation and the pumps to be made in future.

Reliability and Leak-Tightness Indices. In selecting seals on the basis of reliability indices, one must keep in mindthat the seal is a component of the pump and the pump is an auxiliary component of the technological system whose relia-bility requirements depend on the reliability requirements of its constituents. Consequently, the overhaul (inter-repair) cyclesof the pump depend on the overhaul cycles of the main equipment.

It is difficult to predict the reliability indices of the pumps and their seals.In domestic pump making, the average service life is determined on the basis of experience of operation with a

fiducial probability of 0.8 [5].If the required service life of the system is 25000 h until stopping for overhaul, the guaranteed service life of the

equipment under use must be 25000 h. If this is not possible, it is necessary to introduce redundancy (standby facilities) forrepair and replacement of the equipment with a shorter service life. Introduction of standby pumps into the technologicalchain raises expenditures, but the advantage is that all assemblies of the pump are backed up. Ideally, the reliability indicesof these assemblies should be equal to or multiples of the reliability indices of the pump. Note that if the fiducial probabili-ty is 0.8 for average service life, introduction of redundancy will allow this index to be raised to 0.96.

Forced idle time (shutdown) causes loss of profit due to production loss and increase in the share of overheadexpenses in the product cost.

In some cases, it is necessary to take account of the expenditures associated with attainment of working conditionsby the system (desired temperature, pressure, vacuum, etc.).

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a b c

a b c

Fig. 1. Schematic diagram of seals of pump shaft: a) GS; b) FS; c) FGS.

Fig. 2. Schematic diagram of double seals: a) GS; b) FS; c) FGS.

Thus, it becomes necessary to choose between risk of loss due to idle time (shutdown) and increase in cost of theequipment due to introduction of redundancy for guaranteeing the desired service life. Minimizing loss by using seals withquick-detachable components is an alternative version for seal selection. In some cases, quick restoration of working fitness(serviceability) of the assembly with a shorter service life is more advisable than waiting for unexpected failure of the cost-ly seal whose replacement requires dismantling of the pump.

The reliability indices of the critical assemblies are determined for specific operation conditions by time taking andcostly bench service-life tests. As an example, we may mention FC for ship building in Russia, which has a service (operat-ing) life of 50000 h guaranteed with a probability of 0.9.

In Table 1, we have listed the reliability indices of seals of various types of shafts of water pumps (ISO 2858)obtained on the basis of experience of long-term operation.

It is important to stress that the service life of seals depends not only on the design and the materials used but alsoon the quality of pump construction (on the degree of coaxial alignment, shaft wobbling, bearing quality, etc.). Also, tem-perature and pressure of the fluid being sealed, starting-stopping conditions, delivery of sealing liquid, and a few other oper-ational factors affect the service life of seals. So, installation in usual water supply pumps of costly seals made for submarinesmay not justify the claimed service life on account of low demands made on precision of pump making and technical oper-ation level.

The leak-tightness of the seal is characterized by the magnitude of leakage. In simple (single-piece) seals of pumpsfor neutral liquids (usually for water), the ecological damage and a part of the cost depend on the product loss. The experi-mentally obtained leakage values and the power consumed by the pump are cited in Table 2.

For complete leak-tightness (leakage of chemically reactive, combustible, fire-prone, and other fluids), double sealswith hydraulic seal are used. Leakage of the sealing liquid occurs both outside and inside the pump. To evaluate such seals,it is necessary to take account of the loss due to internal leakage itself and dilution of the pumped product. A design versionwith hydraulic seal is tandem seal, i.e., two seals placed in sequence (one after the other), between which is fed a buffer liq-

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TABLE 1

Seal type Average nonfailure operation time, h Average service (operating) life, h

GS 1000; 4000* 8000; 12500*

FS 25000; 12500* 25000; 12500**

FGS 3000; 5000* 25000; 36000*

* With packing from graphite fiber.** With low-cost friction pair.

TABLE 2

Seal type Leakage, liters/h Liquid loss, m3/yr Power consumption N, kW Power loss, kWh/yr

GS 35 2542 0.4 33600.51*

FS 0.0020.035 0.020.3 0.08 6720.001*

FGS 0.10.25 0.92 0.08 6720.050.1*

* During idle time.

uid under a pressure which is lower than the pressure of the liquid being sealed. In this case, it is necessary to take accountof the loss of the pumped product and the cost for its utilization.

In pumps pumping abrasive hydraulic fluids use may be made of simple (single-piece) seals, before which sealingwater is fed. The water consumption must be taken into account in the operational costs.

If all three types of seals meet the safety and leak-tightness criteria, the proportion of the expenditures associatedwith the seal assembly in the obtained product is the selection criterion.

Comparative Analysis of Leak-Tightening Cost. The cost of shaft leak-tightening depends on the following:initial cost and service life of the seal, cost of operation and parts for ensuring inter-repair cycle of the technological sys-tem, cost of power consumption, magnitude and cost of leakage, including utilization cost, and consumption and cost ofsealing liquid.

The total cost of leak tightening in the inter-repair cycle of the system is determined by the formula

C = Cs + (Cwq1 + Cen)T + (Cs + Cch)[(T /Ts) 1] + (Cp + Cr) [(T /Tr) (T /Ts)],

where T is the inter-repair time of the technological system, h; Ts is the service life of the seal, h; Tr is the service life untilreplacement of the packings, h; Cs is the cost off the seal; Cp is the cost of the packing; Cr = C0T1 is the cost of replacementof the packing; C0 is the cost of 1 h of operation; T1 is the labor input for replacement of the packing; T2 is the labor inputfor replacement of the seal with dismantling of the pump; Cch = C0T2 is the cost of replacement of the seal, including disas-

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EC China Russia USA

2000

Initial costCost of seal replacementCost of water and powerCost of packing replacement

1000

500

1500

2500

0.3

C, Euro1

2

34 5

6

1 2

3

45 6

12 3

45 6

1

2

3 4

5 6

EC China Russia USA

500

0

C, Euro

1 2

3

4 5 6 1 2

3

4 5 6 1 2

3

4 5 6 1 2

3

4 5 6

Fig. 3. Cost of pump seals (T = 25000 h): 1) GS; 2) GS with graphite fiber packing; 3) FS; 4) FS withlow-cost friction pair; 5) FGS; 6) FGS with graphite fiber packing.

Fig. 4. Cost of pump seals (T = 1500 h). The legends are the same as in Fig. 3.

sembling and assembling of the pump; Ce is the cost of 1 kWh of electric power; q1 is the leakage of the pumped liquid,liters/h; and Cw is the cost of water. The ratios T/Tp and T/Ts are rounded up to the higher whole number.

As an example, let us consider the efficiency of seals for cradle-mounted water (ISO 2858) with a shaft of 48 mmdiameter. The initial data are furnished in Table 3.

The prices of identical seals vary widely from one country to another but the price ratio for the analyzed types ofseals remains roughly the same. For simplicity, the cost of seals and replaced components is taken as equal to arithmeticmean. For specific conditions, more accurate analysis is needed.

The costs of water, electric power, and repair and maintenance (servicing) work are taken as average for the USA,EC countries, Russia, and China (Table 4).

Analysis of the data in Tables 14 and Fig. 3 shows that mechanical face seals (FS) are much more efficient thangland seals (GS) when their service life is comparable with the service life of the technological system. In all cases, FGS ismote effective.

In periodically acting pumps, use of GS is more economic than mechanical face seals with a long service life (Fig. 4).

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TABLE 3

Seal typeCost, Euro Labor input for replacement, h

assembly replaced components packing seal (with dismantling of pump)

GS 40/50* 5/23* 0.8 6FS 150**/500 6

FGS 100/102* 0.3/1.4* 0.6 6

* With packing of graphite fibers.** With low-cost friction pair.

TABLE 4

CountryCost

of electric power, Euro/(kWh) of water, Euro/m3 of repair and servicing, Euro/h

USA 0.055 0.13 30EC 0.17 3 15

Russia 0.03 1 2.5China 0.09 0.2 0.8

TABLE 5

Seal type Ts, h Tr, h Cs, Euro C, Euro

Double GS 7000 500 50 9260Single FGS 16000 2000 120 860Single FS 15000 700 1840

Leakage of sealing liquid qw taken as equal to 0.11 m3/h.

Let us discuss some aspects of operation of plugging seals. The cost of such seals is at least twice as much as thecost of simple (single-piece) seals even without taking account of additional cost of sealing liquid delivery.

In comparing various types of seals, special attention must be paid to leak-tightness of the inner stage of the seal.It has been found that in GS the contact pressure of the packing of the inner stage is practically not controllable, and it isimpossible to offset radial wear of the protective bushing of the shaft without replacement of the whole package. This draw-back is especially significant when the sealed liquid contains abrasive particles. Because of this, in dredging pumps theinner stage is replaced by slot seal. Experience shows that in this case, if the design is correct, use of FGS without deliv-ery of sealing (plugging) water is much more economic because their service life is not less than the service life of the partsof the flow section of the pump. The cost of leak tightening of sanitary pumps for an inter-repair cycle of 25000 h is givenin Table 5. The reliability indices and consumption of plugging (sealing) water are taken on the basis of experience of useof such seals in Russia.

For leak tightening of corrosive liquids, when use of plugging seal is indispensable, double FS is the most effective.The design of plugging seals (back-to-back, tandem, and with delivery of plugging liquid to the center of the friction pair)must be selected with due consideration for the technological peculiarities of production.

Conclusions1. In spite of differences in absolute cost of electric power, water, and labor in different countries, the cost ratio for

various types of seals is qualitatively similar.2. Face gland seals in general-purpose industrial water pumps are 46 times more efficient than gland seals and

1.53 times more efficient than face seals.3. In water pumps the total cost of more costly face seals with long-lasting friction pair is higher than the total cost

of face seals with low-priced friction pair.4. The current operational cost depends on the cost of electric power loss, and the losses resulting from leakage do

not exceed 7% of this cost.5. Use of gland seals is beneficial where the pumps operate periodically for a short time.

REFERENCES

1. J. Gaft, A. Zagorulko, V. Martsinkovsky, and S. Shevchenko, Face packing seals new opportunities for pump rotorhermetic sealing, in: Proc. Sixth Intl. Conf. Fluid Sealing, BHR Group Conference Series Publication, No. 42,335350 (2000).

2. Y. Z. Gaft, Technical and economic analysis of use of various types of seals, Papers at the Fifth Conf. on SealingTechnology, Sumy (1988), pp. 6263.

3. D. A. McKenzie, A comparison of the cost effectiveness of soft packing materials and renewable face mechanicalseals, Pump-Pumpes-Pumpen, No. 62, 518521 (1971).

4. John Crane Inc., Split seal provides cost and reliability benefits, Sealing Technology, p. 4 (January 2005).5. O. V. Yaremenko, Pump Testing [in Russian], Mashinostroenie, Moscow (1976), p. 224.

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