Double-seal pumps

Note Another item often overlooked is the piping, instrumentation, and electrical costs associated with double seal pumps (seal oil pot, piping, pressure controls, and interlocks). The cost is included in the pump estimation procedure. 

Seal failure (leak) alarm (for double seal pumps)... 

First, one pump running in a parallel system tends to suffer from cavitation because operation to the right of the BF.P indicates that the NPSHr of the pump rises drastically. To survive this condition, you should use dual mechanical seals on these pumps. Dual or double mechanical seals can withstand cavitation better than a single seal. There is a discussion on this in the mechanical seal chapter of this book. Many engineers perceive that parallel pumps are problematic becau.se they appear to suffer a lot of premature seal failure. Parallel pumps deserve double seals even if it s only a cold water system. 

The solution is Parallel pumps should have dual or double seals installed to withstand cavitation when one pump is running solo. 

This type of double seal (with back to back faces) is pressurized above the pressure inside the seal chamber (Figure 13-20). It is recommended for toxic, explosive, costly, dangerous, and volatile liquids. It is important to maintain the seal pressure above the pumped pressure inside the seal chamber. 

Double seals require some type of support system. The reason is simple. With two seals mounted onto the same shaft, one seal is the principal or primary seal and the other becomes the secondary or back up seal already installed. If the primary seal is performing its function and sealing the pumped liquid, the. secondary. seal would be running dry, overheat, burn and self-destruct. Then when the crucial moment comes, we won t have a second seal to assume the ftinctions, which was the original reason to consider a dual seal. 

Dry Running Pump Again, no lubrication or dissipation of hear. Remove the heat with a double seal and barrier tank with forced convective flow. 

Note Most of the costs associated with the double seal could be estimated with the piping, electrical, and instrument accounts. However, to insure their inclusion in the estimate, they should be included together with the pumps... 

Published pump efficiencies are generally based on the use of single mechanical seals during efficiency testing. If double seals are specified, the efficiency of the pump will be lower than that shown by the efficiency curves. 

Tests were run on pumps (without impellers) to determine the power consumption of the seals. These were unbalanced single seals, outside seals, and double seals, with carbon on... 

There was no significant difference in power consumption between an unbalanced single inside seal and an outside seal. However, there was an increase in horsepower when double seals were used on pumps, especially those with low-horsepower drivers at four-pole speeds (Table I). 

Fig. 5 shows operating horsepower vs. additional decrease in points of pump efficiency when using a single or double seal, based on performance data that have been converted from 3,550 rpm to 1,750 rpm by use of the affinity laws. 

Liquid-ring pumps require shaft seals. A simple lantern-ring arrangement appears on some models and requires a flow of cooling liquid. Recycled liquid is acceptable in this duty. However, mechanical seals are preferred in most chlorine-handling applications. Double seals with separate supplies of clean fluid are the standard. 

The purified commercial di-n-butyl d-tartrate, m.p. 22°, may be used. It may be prepared by using the procedure described under i o-propyl lactate (Section 111,102). Place a mixture of 75 g. of d-tartaric acid, 10 g. of Zeo-Karb 225/H, 110 g. (136 ml.) of redistilled n-butyl alcohol and 150 ml. of sodium-dried benzene in a 1-litre three-necked flask equipped with a mercury-sealed stirrer, a double surface condenser and an automatic water separator (see Fig. Ill, 126,1). Reflux the mixture with stirring for 10 hours about 21 ml. of water collect in the water separator. FUter off the ion-exchange resin at the pump and wash it with two 30-40 ml. portions of hot benzene. Wash the combined filtrate and washings with two 75 ml. portions of saturated sodium bicarbonate solution, followed by lOu ml. of water, and dry over anhydrous magnesium sulphate. Remove the benzene by distillation under reduced pressure (water pump) and finally distil the residue. Collect the di-n-butyl d-tartrate at 150°/1 5 mm. The yield is 90 g. 

Ethyl phenylethylmalonate. In a dry 500 ml. round-bottomed flask, fitted with a reflux condenser and guard tube, prepare a solution of sodium ethoxide from 7 0 g. of clean sodium and 150 ml. of super dry ethyl alcohol in the usual manner add 1 5 ml. of pure ethyl acetate (dried over anhydrous calcium sulphate) to the solution at 60° and maintain this temperature for 30 minutes. Meanwhile equip a 1 litre threenecked flask with a dropping funnel, a mercury-sealed mechanical stirrer and a double surface reflux condenser the apparatus must be perfectly dry and guard tubes should be inserted in the funnel and condenser respectively. Place a mixture of 74 g. of ethyl phenylmalonate and 60 g. of ethyl iodide in the flask. Heat the apparatus in a bath at 80° and add the sodium ethoxide solution, with stirring, at such a rate that a drop of the reaction mixture when mixed with a drop of phenolphthalein indieator is never more than faintly pink. The addition occupies 2-2 -5 hoius continue the stirring for a fiuther 1 hour at 80°. Allow the flask to cool, equip it for distillation under reduced pressure (water pump) and distil off the alcohol. Add 100 ml. of water to the residue in the flask and extract the ester with three 100 ml. portions of benzene. Dry the combined extracts with anhydrous magnesium sulphate, distil off the benzene at atmospheric pressure and the residue under diminished pressure. C ollect the ethyl phenylethylmalonate at 159-160°/8 mm. The yield is 72 g. 

As an approximation, 50% of the Hst price should be added to account for auxiUaries such as motor, seal, baseplate, and coupling. Furthermore, the material upgrade from ductile iron constmction to stainless steel roughly doubles the price. Pump manufacturers use price bookmultiphers, eg, discounts, to arrive at a selling price. These vary, but as an estimate, a 30% discount is reasonable. U.S. pump manufacturers include the following. 

The bearings are W ater lubricated. The typical materials of construction are carbon, copper alloys, bakelite and ceramics. The mechanical seals, like a double oil seal protected w ith a cap called a Sand Guard, are robust and perfect in sealing the inotor to prevent the entry of pumped... 

There are trade-offs for magnetic drive and canned pumps versus centrifugal pumps with double-mechanical seals. The former have no seals to leak, but need active interlocks to prevent high temperature for temperature sensitive materials. Similarly, diaphragm pumps, that have... 

---