Basic topics of mechanical seals.

Argesanu, Veronica ; Jula, Mihaela ; Laza, Ioan 等

1. INTRODUCTION

The constructive diversification of mechanical seals to a couple of materials according to their destination, depends directly on the allowed speed range, on the pressure inside the sealed space and on the way in which the hydrostatic trimming of the two rings was made, and finally, depends on the operating economic course of activity. (Cristea;s.a. 1973, Mayer, 1977)

The complete adjustment of all the types of mechanical seals implies additional anticorrosive and calorific protection of the elastic pre-tensioned component, the cooling of the surfaces which are in contact, and re-circularization and/or the evacuation of the discharge flow.

All the functional and constructive conditions mentioned above have lead to the generation of facility thermical seals, which, starting from basic elements like rings, arches, that vary from a wide range that covers various particular requirements.

2. CONSTRUCTION AND OPERATION

2.1 Construction

A mechanical seal is made, mainly, from the following components (fig.1):

--A stationary ring (1) and a sliding one with a relative rotation movement (2), one of these rings allowing the axial movement on the shaft compensation of the wear.

--A system of reciprocal pressing system of the two rings--represented in the figure by the cylindrical spring (3) and by the seals represented by the two "o" rings immobile towards the sheaf and the housing.

--The average dimension of the realized clearance h and the axial force verso pressing force Fa, determine a certain pressure behavior inside the clearance that has an influence on lubrication of the contact surfaces, their durability and tightness.

--The compression of the sliding ring by the pressure depends on the h rates between the pressurized surface, Ah, and the sliding surface A, and finally on their mutual position.

[P.sub.a]/p = [A.sub.h]/A = k (1)

Taking in consideration the above proportion we can have the following situations (fig 2):

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

2.2 Operation

--K>1, which means "unsteadily" or " fussy" seals;

--K<=1, which means "uncharged" or " balanced" seals;

--K=0; we have this situation when the contact compression is not influenced by the environment pressure, being obtained only through the force of the spring.

The unbalanced seals, having k>1, that is [p.sub.a] > p, have a more pronounced wear of the sliding rings, but in the same time they have a more stable operation, an experimental lower pressure in the clearance.

According to the data, the losses through the no seals Q, are in inverse proportion with the square of contact pressure p:

Q[p.sup.2] = const. (2)

2.3 Energy Consuption

The energy consumption process is taking place because of the following dissipative sources:

a) Detention (discharge) of the fluid quantity contained inside of the seal--this loose will be generally speaking, neglected.

b) The sliding friction between the mechanical sealed surfaces; it is accepted that these loses can be formulated according to moment [M.sub.fr] [mNm] and the relative angular speed [omega][rad/s], with the help of the following ratio:

[P.sub.fr] = ([DELTA] [P.sub.fr]/[d.sub.[alpha]]) x d x w/100 = [10.sup.-3] x [M.sub.fr] x w [w] (3)

The friction moment which appears at the mechanical seal represents the amount between the moment of friction of the contact surface [M.sub.a] and the moment due to the movement of the seal into the fluid M[.sub.h].

[M.sub.a] = [pi] x [D.sup.2.sub.m] / 2 x b x [p.sub.a] x [mu] (4)

The moment [M.sub.a] is important; it has a bad influence on the thermic charging of sealing and wear of the rings. The component [M.sub.h] has a great importance at high speeds having a positive influence, generally having a cooling effect.

[M.sub.a] = [pi] x [D.sup.2.sub.m] / 2 x b x [p.sub.a] x [mu] (5)

This last loss will be determinated in all the cases. The designer has the opportunity that through the discharge effect to influence the value of pa and therefore this loss. The contracted ring determines a smaller loss, but is a disadvantage, by representing a greater danger of braking. Estimating the losses through the non sealing we can have the following situations:

a) the conditions of mixed friction in the annular tumble

[Q.sub.2] = [pi] x d x ([p.sub.1] [p.sub.2]) x [h.sub.2] x S/ [p.sup.2] (6)

S=interface factor

b) mixed friction conditions in the annular thimble

Q = [c.sub.2] x [pi] d x ([p.sub.1] - [p.sub.2]) x [eta] x [square root of vb]/[p.sup.2] (7)

c) fluid friction conditions in the annular thimble

Q = [c.sub.3] x [pi] x d x ([p.sub.1] - [p.sub.2]) x [square root of [eta]] b[v.sup.3]/[p.sup.3] (8)

The abstraction of the heat is made more often by the sealed medium, excluding the vaporization process in the interstice. The temperature arising inside the interstice is a consequence of the heating produced through the friction, and may produce pronounced detritions, the aerate of the lubricant film that will result in to a that fast growing of friction coefficient and of detritions, in some cases the exceeding of the integration temperature of materials, which can lead as well to a thermic breaking of the rings, reciprocal slider of thermal palling.

Taking in consideration all the condition mentioned above, we can say that rings made of materials with a high thermal conductivity, may have a better behavior in connection with the thermic aspects. The seals used for high temperature have to be reared as much as possible, the temperature from interface is being kept under the critical temperature of vaporization, in order to prevent the appearance of dry friction, anticipating, for example (fig.3) the construction with double mechanical seals. In case the maximum temperature passes the sensitive point of the elastomeric components of the seal, then we have to use metallic water skins (fig.4), having the advantage of a small axial gauge, higher capacities of loading and better flexibility characteristics. (Aithani et al., 2006, Basu & Butler, 2008, Kharitonov, 1973) Regarding the seals for a higher speed of the shafts of the pumps compressors or turbines, the springs are fixed on the armature or we use seals with an intermediate floating ring (fig 5). Secondly, it becomes necessary to mix as less as possible the obdurate liquid (a fluid oil with a pressure greater than that of the exploitation medium) with the exploitation medium, a small energy consumption at obturation (by a small consumption of obturator liquid) as well as by the transference cross section of the obturator liquid in the exploitation liquid. There have been found different solutions like double seals, in whose intermediate space is coursing the obturator liquid (fig 6).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

This answer gives a higher safety against the attrition of inside the fitting. The liquid obturator can be used (2), in the same time (1), for cooling the seal. For a better washing result, the washing section has to be as small as possible, in order that a sufficient speed to draw the solid particles. A small opening of the inside can be obtained in various ways : with carbon, bronze or PTFE rings. In the case of working in environments with stringy suspension, the compression springs have to be protected against blockage (fig.7). (Nagaralli et al., 1997; Mayer, 1961)

[FIGURE 7 OMITTED]

3. CONCLUSION

In conclusion, the particulars that appear in seal problems, the constructive solutions, the right choosing of materials, as well as the usage of special made accessories lead to the diversification of seals types, taking in consideration the functional aspects as well as those of safety in exploitation

4. REFERENCES

Aithani D.; Lockhart H.; Auras R. & Tanprasert K. (2006). "Predicting the Strongest Peelable Seal for 'Easy-Open' Packaging Applications", Journal of Plastic Film and Sheeting

Basu P. & Butler J. (2008). "Studies on the operation of loop-seal in circulating fluidized bed boilers", Mechanical Engineering Department, Dalhousie University, P.O. Box 1000, Halifax, Nova Scotia, Canada

Cristea, V.; s.a. (1973). EtansariET, Bucuresti

Kharitonov V. (1977). "Mechanical seals for vessels with agitators", Chemical and Petroleum Engineering

Mayer, E. (1961). Leakage and Friction of Mechanical Seals Paper E3, England

Mayer, E. (1977). Mechanical Seals Newnes, Butterworths London, Boston

Nagaralli R.; Ramakrishna A. & Varada Rajulu A. (1997). "Physico-Mechanical Properties of Filled Polyethylene Films", Journal of Plastic Film and Sheeting