Technical Support

Can liquid ring vacuum pump creates vacuum up to 760 mm of Hg (G)?

Liquid ring vacuum pumps are versatile machines because they can handle "wet" loads and can operate down to around 1" HgA. They require proper operation and monitoring, however. This article will address several monitoring and troubleshooting situation involving liquid vacuum pumps (Figure 1).

A check valve will prevent a backflow of gases at star-up or shutdown of the system. When choosing a check valve, a low pressure drop type should be considered to minimize pressure loss in the system. A butterfly style check valve normally has the lowest pressure drop.

Typically for pressures of 100mmHg(G) a and less, some sort of anti-cavitations or suction pressure control is needed. The simplest type is a manual bleed valve such as a globe valve. If automatic opening and closing is required, than an inexpensive spring operated diaphragm valve can be used. In this design, the suction pressure opposes the spring. When the vacuum in the sensing line pulls the diaphragm down, air will be bled in to restore the pressure. Barometric pressure will affect the set point range, so periodic adjustment may be required. By bleeding air in on reduced load condition, this device will prevent the pump from cavitations, but it will not accurately control suction pressure.

For accurate control of suction pressure, an absolute pressure transmitter, a pressure controller and a recycle valve (typically a pneumatic diaphragm operated globe valve) should be considered. This combination will maintain a constant pressure by recycling non-condensable from the separator discharge to the vacuum pump suction.

It is a good idea to have a vacuum vapor and seal liquid into the separator tank. By centrifugal action and the force of gravity, the vapor and liquid are separated, with the vapor exiting the top and the liquid discharged at the bottom of the separator tank. In a recirculated seal fluid arrangement seal liquid is cooled and then recirculated to the pump. A level gauge should always be installed on the separator tank. Visible indication of the contents of a closed vessel is good engineering practice.

The liquid ring pump discharges vapor and seal liquid into the separator tank. By centrifugal action and the force of gravity, the vapor and liquid are separated, with the vapor exiting the top and the liquid discharging at the bottom of the separator tank. In a recirculated seal fluid arrangement seal liquid is cooled and then recirculated to the pump. A level gauge should always be installed on the separator tank. Visible indication of the contents of a closed vessel is good engineering practice.

Level gauges typically used are the tubular glass type and the reflex glass type. A tubular glass gauge can be used fro water or other non-hazardous fluids. If the fluid in the separator tank is flammable or hazardous, then a reflex gauge should be used over a tubular glass type. This will provide better protection from break-age, which would result in fluid in the tank leaking out.

The liquid ring pump usually handles a "wet" or condensable load. Vapors will condense in the pump and be discharged into the separator. This will produce excess fluid in the separator. The simplest way to maintain a constant level in the separator is to have an overflow connection on the separator tank. This will drain excess fluid. If make-up needed, hen usually a manual valve is installed. If automatic make-up is required, a float operated valve can be used. This, along with the overflow connection, will maintain a constant level in separator. If remote monitoring and automatic control is required, then level switches and automatic on/off valves can be used. Another option is to use a displacer type level controller or transmitter connected to automatic throttling valves.

It is a good idea on recirculated seal liquid systems to have manual drain and fill/make-up valve. The fill/make-up valves is used during initial filling of the ring pump system and also during normal operation for any make-up required.

Now that the level is maintained, the amount of flow to the vacuum pump needs to be controlled and/or monitored. On recirculated seal fluid arrangements with recirculation pumps, a globe valve or orifice is required to regulate flow to the pump. A compound gauge, bourdon tube type, is useful when trying to regulate the flow. The pump manufacturer will recommend the seal liquid pressure that should be observed on the compound gauge for proper flow of the seal liquid. On recirculated seal fluid arrangements without recirculation pumps, only a manual shutoff valve (usually a ball or gate valve) is required for maintenance. This will keep the pressure drop to a minimum. A compound gauge would not be needed since the liquid ring pump will pull the amount of fluid it needs to satisfy itself. No regulation or pressure reading is normally required.

The vacuum pump requires a certain of seal liquid to operate at the design pressure. Not enough liquid may result in insufficient vacuum, reduced capacity or possible cavitations of the pump. To alert the user if seal liquid flow is low, a flow switch can be installed. The paddle/vane type is the most common. This has a large pressure drop associated with it. On recirculated systems with no recirculation pumps, pressure drop needs to be kept to minimum for proper flow of the seal fluid. A thermal dispersion flow switch with low pressure drop can be used for monitoring the seal fluid flow in this instance.

Sight flow indicators or rota meters can also be used for visual assurance of flow. The rota meter can be used to regulate flow and indicate the quantity of flow. (Note that recirculated systems without a recirculation pump, a rota meter should not be used due to the pressure drop associated with it.)

A seal liquid color (heat exchanger) is needed to remove the heat of compression, friction and condensation. Different types of exchanges can be used.

For example, plate and frame heat exchanges are economical and best suited for the close temperature approach requires; shell and tubes are the conventional heat exchanger; and a coiled heat exchanger is compact, saves space and has better heat transfer characteristics vs. a shell and tube style.

If a liquid cooled heat exchanger is not possible, then an air cooled model can be considered.

The temperature of the seal fluid directly affects the vacuum at which the pump can operate. A good troubleshooting device to have installed on the system is a bi-metal thermometer. This will indicate the temperature of the seal fluid so it can be compared to design. If remote monitoring or shutdown is requires, then a temperature transmitter, RTD or thermocouple can be installed.

Liquid ring pumps require a shaft seal flush. Packing glands, single mechanical seals and double mechanical seals all require a clean source of flush fluid to cool and lubricate the shaft seals. For packing glands, the flush fluid enters the stuffing box and is dispersed across the glands. The packing will have some dripping associated with it, which means it is being properly lubricated and cooled. For single mechanical seals, the flush fluid lubricates and cools the seal faces, and on many liquid ring pumps, the flush fluid will be discharged directly inside the pump, mixing with the liquid ring. For this reason the flush fluid and liquid ring fluid should be compatible. (Normally they are the same fluid). The flush fluid on double mechanical seals and then leaves it through a separate connection. There is no mixing or contact of the mechanical seal flush fluid and the liquid for double seals unless there is a flush fluid. Many flush plans are available. API (American Petroleum Institute) publishes some of these.

Y-type strainers can be used to remove large particles. However, periodic inspection will be needed to insure that the strainer is not clogged. On systems without a recirculation pump installed, a y-type strainer should be only be used on the make-up fluid line to filter out any particles. If it were to become clogged on the seal fluid line, then it would starve the pump of seal liquid and could cause damage. If the process is dirty, then a strainer ahead of the vacuum pump system should be installed.

With proper indicating, regulating and monitoring accessories installed, the liquid ring pump system will operate trouble free for years. This has been a general over-view of the types of controls typically used or required for proper operation and monitoring of a liquid ring vacuum pump system.

Additionally, the liquid ring's scrubbing action reduces emission as compared to the discharge from the atmospheric-stage ejector, even with a shell-and-tube after condenser. However, this reduction depends on the molecular weight and vapor pressure of the organic vapors.

Design Considerations

While energy conservation is a key factor in the design of a vacuum system, there are several other specifics that should be taken into consideration. Among these are:

System performance and flexibility
Optimum interstage pressure
Approach temperature
Percent non-condensable
Liquid-ring pump size limits
Seal liquid fluid choices
Seal liquid temperature and rise
Single- or two-stage pump
Sealing methods (packing or mechanical seals)
Selection of pump materials for process conditions
Packaging by manufacturer to minimize installation time
Use of condensate hot well(s) to separate hydrocarbons or organics from recirculated seal water
Existing system revamp, including properly matching up existing ejectors with new pumps

The design of a vacuum system requires careful planning and analysis. Energy savings must be a primary consideration in this process.

Installations that utilize a combination of steam ejectors and liquid ring pumps can reduce energy consumption dramatically, providing chemical facilities with increased profits and reducing the cost of product for their customers.

How to Size Liquid Ring Vacuum Ring

Liquid ring vacuum pumps help solve the problem of handling "wet" gas mixtures.

BY JOE ALIASSO

Liquid ring vacuum pumps are commonly used to handle "wet" gas mixtures (i.e., mixtures containing condensable vapors). The moisture present in such mixtures can wash away the lubricating oil in traditional "dry" oil lubricated pumps, possibly leading to premature failure.

As the impeller of the vacuum pump rotates, it throws water by centrifugal force to form a liquid ring concentric with the periphery of the casing which does the work of compression. The eccentric mounting if the impeller with respect to the casing results in increased spacing between the impeller bladed at the inlet port and decreased spacing towards the outlet port (Figure 1). As gas enters the inlet port, it is trapped between the impeller rotates, the liquid ring compress the gas and forces it out the outlet port.

The information needed to accurately size a liquid ring vacuum pump includes:

Inlet pressure, usually expressed in mm HgA
Intel temperature
Mass flow rate, usually expressed in LB/hr and the molecular weight of fluid components
Vapor pressure data for each fluid component
Seal fluid data, if other than water: specific gravity, specific heat, viscosity thermal conductivity, molecular weight and vapor pressure data
Temperature of the seal fluid or cooling ware
Discharge pressure, usually expressed in PSIG

Each of these factors influences the sizing of the liquid ring vacuum pump.

Liquid ring vacuum pump manufacturers are able to determine if cavitation can result from a particular application. Cavitation occurs as a result of rapid boiling of the seal liquid. During boiling, bubbles from in the liquid and seek to escape. As the bubbles rise in the liquid, they are subjected to higher pressure zones and can collapse. The void of space that is lift the bubbles collapse in instantly failed with liquid. This phenomenon leads to extensive erosion or pitting of the pump internals.

Cavitation can be detected by a rumbling noise that sounds like marbles rolling around inside the pump. This sound should not be confused with that occurring as a result of "water hammer." If the seal fluid vapor pressure is near the operating inlet pressure of the vacuum pump, then the sound heard is most likely from cavitation.

If the liquid ring pump is disassembled and the impellers and port plates are pitted, cavitation is evidently occurring. A liquid ring pump with flat port plates can be easily machined and reassembled. In contrast, a liquid ring pump with conical ports is more difficult to machine because of the taper and angle of the cone.

Possible ways to eliminate cavitation are:

Use a colder seal fluid. This will lower the vapor pressure of the seal fluid and keep it from flashing.
Use a seal fluid with a lower vapor pressure. This will prevent the seal fluid from flashing and causing cavitation. The seal fluid should be compatible with the process gas mixture and the material of construction. Certain accessories may need to be re-designed to operate with this different seal fluid.
Increase the inlet operating pressure (if this pressure will be tolerated by the process) beyond the range of cavitation.
Check the loading and operating pressure. If the load is less than design, the pump may be operating at a lower absolute pressure, which may lead to boiling of the seal liquid and cause cavitation. A vacuum relief valve or air bleed valve can be used to introduce additional load so the pump will be able to operate closed to the design pressure.
Install a booster upstream of the liquid ring vacuum pump. The booster will compress and raise the absolute pressure of the vacuum pump suction. There are many types of boosters, but the simplest type is an ejector that has no moving parts. An ejector works by using pressure energy of the motive to increase the velocity of the entrained gas load. This produces a higher absolute pressure at the outlet of the ejector. Two types of ejectors are a stream are a ejector which uses pressurized stream, and an atmospheric air jet which uses atmospheric air as the motive.
Increase the throughputs of seal fluid, if possible. This will decrease the temperature rise of the liquid ring pump and lower the vapor pressure of the seal fluid.

Many liquid ring vacuum pump manufacturers publish dry air curves with 60° F seal water (Figure 2). From these curves, corrections are made to take into account actual operating conditions. When a seal fluid other than water is used, the liquid ring vacuum pump manufacturers will require the specific gravity, specific heat, viscosity, molecular weight, and vapor pressure data to accurately correct the dry the air curves.

By knowing the vapor pressure of the seal fluid, the manufacturer is able to check for cavitation and also determine its effect on capacity. If the vapor pressure of the seal fluid is less than that of water, than the dry air capacity of the pump is increased. This is because less of the seal fluid will flash and take up space between the impeller blades. The opposite is true if the seal fluid vapor pressure is greater than that of water.

The operating horsepower can be calculated by knowing the specific gravity and viscosity of the seal fluid. The pump curves are usually based on water as the seal fluid, and then the increase or decrease of operating or brake horsepower is determined. The higher the specific gravity and viscosity, the more brake horsepower the pump requires to operate. If the seal fluid is an oil, then the brake horsepower is usually less than that using water and the opposite is true if it is ethylene glycol.

The type of load that the liquid ring handles affects its capacity. The two basic type are a dry air load and a saturated air mixture.

For a dry air load, the air enters through the inlet port into the impeller bucket where it contacts the seal fluid. The dry air instantly saturates itself by evaporating water from the liquid ring. The water from the liquid ring that is evaporated takes up space in the impeller bucket. This reduces the amount of new incoming air load that the impeller bucket can handle, which may result in the selection of a larger vacuum pump.

For a saturated air mixture, the load enters through the inlet port into the impeller bucket, but if the seal fluid is cooler than the incoming gas mixture, condensing will occur. This allows more room in the impeller bucket to handle addition incoming gas and use of a smaller vacuum pump.

The only way the manufacturer can correctly size the liquid ring vacuum pump is by knowing the individual gas components and properties that make up the mixture. This will allow the manufacturer to make proper correction and choose the best pump.

The discharge pressure of the pump is needed to determine any reduction of pump capacity or any increase in operating horsepower. Most liquid ring vacuum pumps are designed to operate from a vacuum suction to an atmospheric discharge pressure. If the discharge pressure is increased, then more energy or brake horsepower is required to operate at the increased compression range. By changing the discharge pressure, the efficiency of the pump is changed and the capacity of the pump may be reduced to compensate for this greater compression range.

Visual inspection should be performed once the liquid ring pump is up and running. The vacuum level, seal fluid temperature and seal fluid pressure should be checked and compared with the design conditions. The bearing housing should be warm to the touch. A hot bearing housing can indicate a bad or worm bearing, or misalignment.

A liquid ring vacuum pump is a very versatile machine. It can handle "wet" loads and has no metal-to-metal contact. It also acts like a direct contact condenser; it can absorb the heat generated by the compression, friction and condensation of the incoming gas; and it will absorb and wash out any contaminants entrained in the gas. In split of this versatility, however, sizing and selection of the most economical system still requires full information concerning not only what fluids it will handle, but how it will be operated.