5.6 Choice of pump seal
Choice of pump seal
The choice of a pump seal comprises many aspects to evaluate and without knowledge and experience it is very difficult to overview the entire picture. For most cases, a number of consequences for solution A must be compared with possible dito for choice B or C. No distinct rules can be given for type choice of a shaft seal. In many cases a pump can be equipped with a mechanical seal or a soft packing entirely in accordance with the purchaser wishes. The choice should, however, be made in with actual local conditions such as how the leakage can be handled and above all the manner in which pump maintenance is to be carried out. For many cases of pump seal selection, the number of aspects to consider are so many that an uninitiated person have to rely on luck to make a successful selection. The risk of failure is imminent and even if all necessary information has been evaluated it is recommended to contact a seal manufacturer for a second opinion before a decision is taken.
The prime cost for stuffing box or soft packing material is relatively low compared with the prime cost for a mechanical seal. The prime cost is, however, not decisive and the total cost is largely determined by a number of secondary costs associated with leakage and auxiliary systems. If the quantities of leakage and costs per year are determined it can be seen that the differences between different types of seals can be considerable. This can result in very large costs if a barrier liquid is required, or if the liquid is dangerous and volatile. The maintenance costs for a mechanical seal can be seen to be lower than for a stuffing box or soft packing and the additional costs for a mechanical seal when purchasing a pump are usually recovered after one year’s operation. Table 5.6a gives a hint of Mechanical Seal versus Stuffing box life cycle cost comparison:
Point of evaluation | Mechanical Seal | Stuffing box |
Initial cost | Relative high | Low |
Leakage | Little | Large |
Operating cost | Low | Quite high |
Shaft wear | No | Can be severe |
Maintenance/Adjustment | No | Regular |
Reliability | Very good | Good |
Friction loss | Same | Same |
Applications | Rotating shafts | Rotating and axial movement |
Table 5.6a Mechanical Seal versus Stuffing box LCC comparison
Check list for pump shaft seals selection
Pump:
- Type of pump
- Size and dimensions
- Stability
- Location
Medium:
- Contaminants
- Viscosity
- Vapor pressure
- Specific gravity
- Crystallisation
- Polymerisation
- Toxicity
- Explosive
- pH value
- Radio-active
Temperature
- At the seal
- Minimum/Maximum
- Variations
- Temperature shocks
Pressure
- At the seal
- Minimum/Maximum
- Variations
- Pressure shocks
Shaft
- Diameter, tolerance
- Material, surface finish
- Speed
- Direction of rotation
- Rigidity, deflection
- Run-out, rotational and axial movement
- Vibrations
Housing
- Radial cavity
- Axial cavity
- Number of fixing screws and positions
Other factors
- Tightness requirements
- Hygienic requirements
- Standard requirements
- Expected service life
- Access to barrier fluid
Seal selection
The factor pressure times speed or the pv-factor, in accordance with the Pump Seal section Operating Conditions, is the first technical point to consider when selecting a seal, see figure 5.6a. This table, however, does not take into account the liquid temperature or, more importantly, the temperature difference from the vapour temperature, in accordance with the ΔT concept explained under Temperature aspects in mechanical seals at Mechanical Seals, which is normally used in the case of mechanical seals and has in principle a similar effect on all shaft seals. In the case of pv-factors above approximately 10 for soft packings stuffing box and above approximately 50 for mechanical seals, the frictional heat generated means that the highest liquid temperature is determined to a large extent by the removal of heat through leakage or through extra cooling.
Standardization
The range of seals, both soft packing stuffing box and mechanical seals, is extensive, even too large to be grasped by the pump user. If one then multiplies that with the number of shaft diameters and material combinations then there is clearly a requirement for some form of standardization.
For soft packings and mechanical seals there is an international standard ISO 3069 which comprises shaft and box dimensions.
For mechanical seals there is an internationally acceptable pump seal standard ISO 21049 where certain standardized dimensions make it possible to replace seals of different makes on the assumption that the pump manufacturer has taken account of the maximum permissible seal length and has not adopted a special manufacturer shorter inbuilt length.