Liquid Ring Compressor/Pump

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yesyen Guru Join Date: Jul 2006 Location: India-Chennai. Posts: 722 Good Answers: 30	Liquid Ring Compressor/Pump 10/03/2010 1:49 PM
	In Centrifugal Pumps, the effect of cavitation is well-established phenomena. It is well established that the liquid expands to gas at the lowest possible pressure around the eye of the impeller. On the way of its travel the gas globules bust back to liquid and cause pitting marks. This region of erosion could be along the vane until the tip, depending upon the pressure buildup. I am trying to visualize a corresponding situation in the case of a Liquid Ring Compressor/Pump. My questions are: In Liquid Ring Compressor/Pump, the existence of low pressure cannot be eliminated because that is the very purpose of this equipment. In centrifugal pumps, by taking necessary cares, cavitation could be eliminated. But in vacuum equipment cavitation and its effects look to be unavoidable, am I right? In Liquid Ring Compressor/Pump, low pressure (vacuum) and (relatively) high pressure are successively happening between every consecutive vanes of rotor. So substantial vapor (of liquid ring) has to generate and condense in these vane cavities. Such vapor volume should directly reduce the capacity (ability to generate vacuum) of the machine. If this is true, can we achieve a high vacuum by changing to high vapor pressure liquid from low vapor pressure liquid? __________________ A picture worth thousand words: needless to say if it is animated.
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#2 "Re: Liquid Ring Compressor/Pump" by ducon on 10/03/2010 4:24 PM (score 2)

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#5 "Re: Liquid Ring Compressor/Pump" by Kaisan on 10/18/2010 8:00 AM (score 1)

ducon Guru Join Date: Dec 2006 Location: Hop around Toronto, New York & Karachi Posts: 1876 Good Answers: 19	#1 Re: Liquid Ring Compressor/Pump 10/03/2010 4:15 PM
	Yesyen long time no see __________________ I not only use all the brains that I have, but all that I can borrow. Woodrow Wilson
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Re: Liquid Ring Compressor/Pump

10/03/2010 4:24 PM

While there are many vacuum pumps that can achieve deep vacuum, the liquid ring pump is particularly well suited for soil vapor extraction. First, it can pull deep vacuum so that the benefits described above can be attained.

In addition, liquid ring pumps can handle the slugs of water, soil particles, contaminants and dissolved organics that are likely to be pulled into the pump in this application. Internally lubricated pumps, like rotary vane and rotary screw pumps, are designed for clean service and simply can not be expected to operate for any length of time in this type of environment. While the risk of pulling something into an internally lubricated pump can be reduced by employing a knock-out pot and using a series of filters, it can not be eliminated.

Lastly, liquid ring pumps are exceptionally quiet. They operate at less than 85 DbA and are not creating a noise disturbance to the surrounding area.

When most people think of liquid ring they think of a pump sealed with water. One of the biggest criticisms of liquid ring pumps is that they require vast amounts of cool water in order to pull and maintain deep vacuum. This is because water has a fairly high vapor pressure. If water is used as a seal fluid, capacity drops as the water temperature rises because the sealant vaporizes, reducing the potential capacity of the pump.

Another problem with using water as a seal fluid is that the vacuum system's reliability is limited by the quality of the water used. Typical of remediation sites, is that the only available water is hard in minerals. Hard water leads to mineral plating inside the pump which both hampers performance and requires frequent maintenance. If you use water high in minerals, you should expect the liquidring vacuum pump to have problems and frequent shut-downs (after a year of operation with a competitor's water sealed system, Xerox typically saw 50% up-time, or in other words 50% of the time it was not running).

Humidity is also a problem in this application because high humidity in the discharge gas stream reduces the effectiveness of carbon adsorption and incineration equipment. By using a water sealed vacuum system, you actually add to humidity in the discharge stream, typically approaching 100% RH.

By eliminating water as the sealant, you no longer contribute to the humidity of the discharge gas; maintenance and downtime are reduced. In addition, we can incorporate a humidity control option on the system to provide cool (100o-120o), dry (<40% RH) effluent to the carbon beds.

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Kaisan Power-User Join Date: Nov 2009 Posts: 296 Good Answers: 27	#3 Re: Liquid Ring Compressor/Pump 10/04/2010 9:23 AM
	Cavitation is and should be avoided in liquid ring vacuum pumps. The vapour pressure of the seal liquid (at pumping temperature) and the vacuum achieved governs when cavitation will occur. All of these must be taken into account when specifying the seal liquid and the achievable (allowable) vacuum. Failure to avoid cavitation can result in very severe cavitation damage in a remarkably short space of time. __________________ I have no special talent. I am only passionately curious. Albert E
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yesyen Guru Join Date: Jul 2006 Location: India-Chennai. Posts: 722 Good Answers: 30	#4 In reply to #3 Re: Liquid Ring Compressor/Pump 10/15/2010 9:48 AM
	"Cavitation is and should be avoided in liquid ring vacuum pumps." Agreed, but how? Take a most common application, high vacuum pump with ordinary water as sealing liquid. High vacuum around rotor internal have happen and water would bust to vapour, unless it is very cool, right. Normally the excess heat only remover in a recirculation system and water at room temperature is admitted in 'once through system'. "Failure to avoid cavitation can result in very severe cavitation damage in a remarkably short space of time." That is my question too. Cavitation marks are seen around the impeller eyes in centrifugal pumps. I wanted to discuss, in vacuum pumps where the such cavitation marks are predominantly seen. How to avoid it. I expect more answers from our expert friends. But this discussion did not take off that well... __________________ A picture worth thousand words: needless to say if it is animated.
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Kaisan Power-User Join Date: Nov 2009 Posts: 296 Good Answers: 27	#5 In reply to #4 Re: Liquid Ring Compressor/Pump 10/18/2010 8:00 AM
	Yesyen, which part of my answer was so hard to understand? I repeat, "The vapour pressure of the seal liquid (at pumping temperature) and the vacuum achieved governs when cavitation will occur. All of these must be taken into account when specifying the seal liquid and the achievable (allowable) vacuum." High vacuum around rotor internal have happen and water would bust to vapour, unless it is very cool, right. Right - in this case the temperature of the water has to be maintained below the saturation point at the inlet pressure, otherwise cavitation will occur. This article should answer your question in more detail, including some pictures of cavitation damage. Note that the introduction of non-condensible gas, ie air, via a vacuum relief valve is one common way to prevent cavitation and subsequent damage. (Well explained in the linked paper). __________________ I have no special talent. I am only passionately curious. Albert E
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yesyen Guru Join Date: Jul 2006 Location: India-Chennai. Posts: 722 Good Answers: 30	#6 In reply to #5 Re: Liquid Ring Compressor/Pump 10/18/2010 9:58 PM
	Excellent explanation Kaisan, thanks. The linked article is of great informations and the details therein raise some more questions in my mind. I'm trying to correlate your suggestions and the content of the article with my experience on LRVPs. I specifically want to discuss and distinguish the cavitation marks and erosion marks (due to unavoidable turbulent flow in and around the rotor internal) of vacuum pumps used in air and Chlorine service. Let me consolidate my queries and get back soon. __________________ A picture worth thousand words: needless to say if it is animated.
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