Air Leaks in Falling Film and Forced Circulation Evaporators running under Vacuum Conditions
- Posted on 7th December 2013
- in Falling Film Evaporators
- by Jan de Geest
GLM Hydro Evaporator Problem Solving: – In over 25 years of troubleshooting and consulting evaporator systems, I cannot even begin to tell how many times an air-leak or issue with the de-aeration system in the evaporation system was the cause of capacity- or performance fluctuation problems. Some of my customers are very capable of tracing leaks (educated by experience) and some are not. It is an art in itself to find air-leaks especially those that are hidden under the insulation of the equipment.
Water fill test is no vacuum test
First of all: a pressure- or water fill- test is, in my opinion, not the same as a vacuum test. The direction of the air in a pressure test is from inside the evaporator to atmosphere, as where in a vacuum the air leaks in the opposite direction, from the outside in.
To do a water fill test some parts have to be closed off. For example, if there is an overpressure protection safety valve or safety ball, that connection has to be closed off to be able to fill the system above the safety devices’ connection point. Many times a customer was not able to find the vacuum leak themselves due to the closing off of the safety valves. When I came over to support one of the safety valves turned out to be the culprit since this devise was locked up to be able to perform the water test. Once the system was back under vacuum conditions the pressure safety was leaking in air. A simple bad seal was the problem.
Other incidents that are difficult to locate by a hydrostatic test are leaks in pump seals or leaks in check valves in the condensate or concentrate pump discharge line. For example, when your installations runs a low condensate level and the discharge line of the condensate pump is not submerged at the end of the line, it can happen that air is pulled back into the system via the discharge through a faulty check valve.
I am not claiming a water fill test is not a good idea, it has its own time and purpose. However, it is definitely not always conclusive in ruling out an air leak. Safety valves, pump seals, check valves can be sources of leakage which are difficult to find with a hydrostatic test. A leak in a pump seal such as a double mechanical seal, is easier to locate in an evaporator running under vacuum with the seal water turned on. Normally there is a discharge of seal water from the seal into a drain, however in case of a leaking seal the seal water will be sucked in to the pump house. This leakage can not be detected when performing a water fill test.
Air leak locating
To locate the section where there is an air leak in the system, it can help to measure the shell pressure and shell temperature. Running on water the vapor pressure and vapor temperature should be corresponding to the saturated steam table. If the temperature is to low there might be an air leak problem. An ultrasonic tester can provide great results in the quest for leaks. It will take some practice. Leaks in air-lines around the system should be eliminated to avoid false detection, but once mastered this technique, it will give you a reliable test under vacuum that can be done quick, efficient and effective.
Besides the vacuum test, I like the old fashioned approach as a primary guidance: feel the lines by touching them or shooting a surface temperature with a laser thermometer. WARNING: Be careful with hot surfaces that do not allow touching, please do not get burned! Cold condensate lines, cold de-aeration lines, cold spots on equipment shells can be a clear indication of air leak or de-aeration problems. Always check condensate levels before testing, in case there is a condensate discharge issue and condensate builds up to a level where it can block the de-aeration connection.
Contact us for more information when your installation has these or any other kind of issues.
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Tags: air, evaporator, falling film, vacuum