Retard formation of solids.
Wash away solids that do form.
Provide cooling during normal operation.
Provide heating before startup.
Nitrogen quenches, based upon general observations, are not as effective as steam for quenching high temperature seals. Product decomposition ("coking") is related to temperature. Not only does coke form more quickly in hot pumps, but it also forms more quickly around seals that run hot because of heavy load or inadequate flushing.
Steam quenches can be used with either rotating seal heads or stationary designs. Quenches on rotating seals, sometimes called a "steam blanket", is not particularly effective because very little steam is circulated within the quench area. Depending upon the type of bushing used, the steam can even be directed towards the pump bearings. A steam quench used with a stationary design, such as the Type 1604 (metal bellows seal), is more effective. The steam must enter underneath the bellows assembly, between the bellows and the anti-coking baffle, and is guided around the seal to wash away the leakage from the seal faces.
Care should be taken to make the drain port as accessible as possible with as large a "drill-through" as possible to prevent the drain hole in the gland from clogging up with coke. On a design like the Type 1604, if a quench is not going to be used then the baffle should be removed or modified as this will provide additional clearances to counteract the accumulation of solids.
Determining the Quench Rate
Four considerations determine the recommended quench rate:
- Is a quench required to improve MTBPM?
- Minimum rate to purge the quench volume of the gland.
- Minimum rate based on velocity to wash away leakage.
- Minimum rate for cooling leakage below decomposition temperature.
Is a quench required to improve MTBPM?
For high temperature hydrocarbon services, the general guideline is to apply a steam quench if the pumping temperature is above 350-deg F. The relative effectiveness depends upon many variables, but quenches used on lower temperature services have a reduced effect on extending MTBPM, other things being equal.
Minimum Rate based on Purging
If a quench is to be applied, then the minimum quench rate can be thought of as a purge. In that case, the minimum rate is a function of the volume being purged and the leakage being diluted. For typical seal gland plates and a contingency plan for high leakage rates, dilution of leakage usually governs.
Minimum Quench Rate Based on Washing
Another consideration is that the quench should wash away the leakage. This is based upon the quench rate with a certain velocity thru the quench area. The velocity should be in the range of 10-fps to 15-fps through the flow area to be effective. This consideration may call for more quench than the consideration for purging.
Minimum Quench Rate Based on Cooling
Steam is usually readily available in plants and the flow rates are typically not regulated very closely due to the availability. This is also due, in part, to the cost versus other quench media. The relative cost of quench media is:
Water = 1 (datum) Steam = 0.005
Plant nitrogen = 0.006 Bottled nitrogen = 1.4
The cooling effect of gases such as steam and nitrogen on the face temperature of hot seals is small. The order of magnitude is less than 500-btu/hr removed from the seal faces. If the quench rate is too small, the temperature of the quench will heat up to nearly the pump temperature and allow decomposition and coking to occur. To prevent this, the average temperature in the quench volume can be estimated from an energy balance using the seal leakage rate, quench flow rate and heat soak from the surrounding metal. By constraining this average temperature to be less than some critical "coking" temperature, the quench rate can be computed.
Recommended Quench Rate
After all the above considerations, the recommended quench rate is the largest of the values. For most pump seals the recommendation can be simplified in the following table.
Quench rates for typical high temperature pumps. The rate is volume per inch of seal size.
Controlling the Quench
The recommended quench rates are low enough that the flow rate may be somewhat difficult to control with any accuracy. There is little need for precision, especially with steam. Table 3 shows the approximate flow rate through a simple orifice, for steam and nitrogen.
Approximate flow rates through orifices. (ACFM = Actual Cubic Feet per Minute)