Blower Systems Configuration
The best orientation for a rotary positive displacement blower is in a horizontal configuration. The horizontal configuration can be described as vertical airflow or you can draw a line between the shaft ends. If the line is horizontal it is a horizontal unit. If the line is vertical, it is a vertical unit.
There are several reasons why the horizontal configuration is preferred:
- The oil reservoir volume is greater allowing for more oil to be circulated to the bearings and gears. This also extends the life of the lubricant since there is more of it and therefore takes longer to degrade.
- If water or material is present in the system it can “drain” or “flow” out the bottom once the unit comes to a stop. This also permits water to drain if condensate forms after a unit is shut off.
- There is a lower probability of pipe strain on the Units since piping must first be attached in a vertical orientation. Pipe strain can deflect the cylinder causing the impellers to rub the casing
Base and Supporting Frame Design
A blower is a precision machine. Although they are rugged and can stand a great deal of abuse, if they are not installed properly, they may operate for a short period or not at all. The base is an important first step to a well-designed installation. First the base must be rigid. Robust structural members are required to insure that the frame has minimal flex to limit the possibility of deflection. The blower must be mounted on a flat surface to minimize any twist that may be introduced to the blower when fastening on surfaces that are not level. The surface should be flat within 0.002″ to insure that a deflection in the cylinder does not occur. Shims may be used to insure the unit is installed within this specification. One can check for excessive pipe strain by loosening one bolt of the blower after it is installed. If a 0.002″ shim can pass under the foot, shim must be added. This process can be performed under each foot.
You must also be careful in base design to avoid a harmonic excitation frequency. In some cases it has been seen that the blower at certain or all load points excites a base. Harmonic excitation may be suspected when the blower is seen to shake violently or when a supporting member seems to visually distort during unit operation. This problem can typically be remedied by adding additional bracing or supports to the frame.
Blower systems bases can be classified as elevated, non-elevated or sole plates. An elevated base is sometimes referred to as a “table top” base. Non-elevated bases are typically formed steel or a structural steel weldment mounted directly to a foundation. Soleplates are typically steel plates mounted in a concrete pad. Elevated bases are typically used in systems below 150 HP; however, systems have been built with much higher horsepower in this configuration. Non-elevated bases are typically larger machines in the vertical (side inlet and discharge) configuration. Soleplates are usually reserved for installations requiring 500 HP or greater.
An ideal non-elevated base would have a mass of 2.5 times or greater weight when compared to the weight installed on it. We call this a seismic base. A seismic base provides the stiffness necessary to prevent deflection and also limits structural born vibration. If the unit is mounted on a mezzanine, a seismic base can limit the vibration transmitted to the supporting structure. The seismic base is usually constructed with structural steel that is filled with concrete. Rebar is usually welded into the frame to provide additional support for the concrete. The base is then typically isolated from the structure by the use of spring type isolators.
Isolate the blower from excessive pipe stress. Manufactures recommend that there should be limited flange or connection loading on the blower. Make sure that a flexible connection or expansion joint is installed at the blower interface point. If it is not possible to install at the interface, the closest possible point should be considered. Excessive flange loading will distort the case and cause the impeller to contact the case. This condition will cause premature failure or lock the unit down so that it will not operate or rotate freely. This condition can be easily seen if the unit turns freely prior to installation but will not rotate after it is installed.
Also remember that the gas heats up as it passes through the blower or vacuum pump. This will cause thermal expansion. If a flexible joint is not provided and assuming the unit is securely fastened to the base, the thermal growth will cause the unit to bind. Make sure that the pipe has some degree of freedom to compensate for this growth. All piping should be well supported such that the loads are isolated from the blower package. The optimal position of the expansion joint is at the blower connection interface. Blower accessories such as silencers are typically not designed to carry the weight of a piping system.
Drive alignment is an important factor in the longevity of a blower. V-Belt drive and flexible coupled are the most common forms of drive. Our experience has been that v-belt driven blower systems are the most prevalent. Important considerations when v-belts are used are as follows:
- Check the overhung load of the blower and motor to insure that the maximum limits are not exceeded. Overhung load is the pull exerted on the blower shaft. Refer to the operating manual of the unit for maximum limitations. Excessive load can dramatically shorten the life the drive bearing. In extreme cases it may cause the shaft to fail at the bearing. Typically, overhung load problems can be avoided by using a moderately sized sheave on the blower. Based on a load calculation, smaller sheaves cause greater overhung load for a specific ratio then a larger diameter sheave.
- Confirm that the belts are aligned properly. Typically, sheaves should be no more then 1/16″ out of alignment, while 1/32″ is preferred. Sheave misalignment may cause premature bearing failure as well as cause loss of end clearance in the blower.
- Proper belt tension is important for the life of the blower bearings as well as life of the belt. After a new set of belts is installed, they should be retensioned after 24 hours of operation since they tend to stretch. If the belts are tensioned too tightly they can cause an overhung load problem and premature bearing or shaft failure.
- The sheaves should be mounted as close to the blower drive cover as possible to minimize overhung load. Close is considered to be ¼” to ⅜” on the smaller units and approximately ½” on the larger units.
- V-belts are typically designed with a minimum 1.4 service factor. Verify that this minimum exists. The service factor is based on the maximum continuous horsepower the belts are rated at divided by the actual horsepower requirement of the blower.
Protective Devices for Blower Systems
Typical protective devices utilized for blower protection are as follows:
- Mechanical relief valve
- Temperature switch
- Pressure switch
- Vacuum switch
- Differential Temperature Switch
The mechanical relief valve is the most commonly used protective device. The two most common types are a weighted and spring type. The weighted type is typically installed in a vertical orientation and operates as a piston. Weights are added or removed from the valve body to adjust the set point pressure. The spring type relief valve is adjusted by changing the compression setting of a spring. Spring type is used for both pressure and vacuum installations. The preferred orientation is vertical, however, positions from 0 to 360 degrees can be used. If the valve is mounted in a position other than vertical, beware of possible wear on the valve stem. If a spring type relief valve is used for vacuum, be certain it is installed properly. I have encountered many installations where it is piped to the wrong port and does not provide protection for the unit.
The relief valve should be located such that there are no restrictive devices between the valve and the blower connection. Restrictive devices include; butterfly valves, check valves, filters, bag houses and isolation valves. The closer the valve is located to the blower connection, typically the better. One potential problem is that the pulses that the blower generates can cause premature failure of the valve. The valve can be located after a silencer or on the side of the silencer after one dampening chamber to extend the life of the valve. This location should not be considered if there is a substantial amount of carry over which could plug the silencer
Relief valve operation should be included on your normal maintenance checklist to insure that the blower is adequately protected. The set point of the relief valve should be set such that the set point is equal to or less than the maximum allowable pressure of the blower. Most relief valves have an “accumulation ” factor that must be considered. The valve may not “crack open” to begin the relieving process until 10 to 20% over set point pressure. Most blowers can tolerate this excessive pressure over their design point for a moment. The best solution to a set point is to set it to within 1 or 2 psi of the maximum expected process pressure. This way the blower is protected and the process is protected from over pressure related problems. The relief valve is not a control valve and should not be used in this manner.
Vacuum relief valves should be set to within 1 to 2″Hgg of the set point. Please bare in mind that excessive vacuum can cause a rapid temperature rise within the blower.
A discharge temperature switch is a good control based safety device for constant inlet temperature applications. The temperature switch protects the blower systems from excessive temperature rise due to over pressurization. The discharge temperature switch is not well suited for varying inlet temperature conditions. This basically applies to any application that utilizes ambient air. This can be illustrated by setting the discharge temperature based on the high ambient condition expected. For example, assume that the switch is based on 100°F ambient; therefore the set point may be 180°F. This allows an 80° temperature rise. However, in the winter when the average ambient temperature is 50°F, the switch allovvs a temperature rise of 120°F. This may exceed the maximum allowable for the blower. You may elect to compromise and base the set point on the yearly average. This may protect the unit, however, nuisance tripping may occur when the ambient temperature increases.
Pressure switches can provide protection for the blower at an economical cost. If placed in the discharge line they can provide protection from excess discharge pressure. The disadvantage of the discharge pressure switch is that it does not take into account inlet pressure losses. Inlet losses can become excessive if there is a clogged filter or if it is in a “push-pull” system and the vacuum level is excessive. A corresponding vacuum switch can be provided on the inlet side but will not take into account discharge conditions. A vacuum switch on the inlet of the blower can monitor inlet filter condition and shutdown the unit should the pressure drop become excessive.
A vacuum switch can be used on the vacuum side of the system in much the same manner as the discharge pressure switch. It also shares the same limitations. If excessive discharge pressure is present, the switch will not sense the condition.
A differential pressure switch offers good protection for a blower. The differential pressure switch monitors the difference between inlet pressure and discharge pressure. If the set point is exceeded, the unit can be shutdown or corrective action can be taken. The primary weakness of the differential pressure switch is in monitoring pressure ratio. Pressure ratio is defined as P2/P1, where P2 is discharge pressure in absolute terms and P1 is inlet pressure in absolute terms. The set point of the switch can be set so that the maximum allowable pressure differential is not exceeded on the specific unit. However, if the ratio becomes excessive, the unit can exceed its maximum allowable temperature rise. This can cause unit failure due to impeller growth.
The differential temperature switch is perhaps the best protective device for a positive displacement rotary blower. An RTD is installed on the inlet side and the discharge side of the blower. The resistance of the device is measured and the low temperature is subtracted from the high temperature and is compared to the set point. This device takes into account the pressure ratio. Therefore the unit is protected if the inlet filter is clogged or the discharge pressure is greater than anticipated. This switch allows the unit to be operated at its maximum with less risk of exceeding the allowable parameters. A differential temperature switch is the most expensive option of the electrical based protective devices, however, it provides the greatest protection of your investment.
Flexible connectors are important to a blower installation. Due to tight tolerances inside the blower, cylinder deflections can cause the unit to lock up or produce a rub on the impellers. Neither of these situations is conducive to long life. A flexible connector isolates the blower from pipe and system mechanical loads. Flexible connectors on both the inlet and discharge connections are the optimal solution. However, a discharge flexible connector is the most important position due to the thermal growth of the unit. If the discharge piping is fixed, the thermal growth can cause the piping to grow and the growth usually impacts the blower. Material selection of the connector is important. It must be rated for the temperature and compatible with the gas handled. Typical flexible connectors are hose type or spool type joints. Single arch expansion joints are common for flanged connections and hose is used for smaller pipe connections. Typically hose type are used for 4″ and smaller connections, however, they have been used up through 10″ pipe size. Common materials for the inlet side are neoprene, butyl or EPDM. Discharge connections are normally EPDM, silicone or stainless steel. EPDM connectors are typically rated for about 350°F at pressures to 25 psig.
Pressure and vacuum gauges are important monitoring instruments for blowers. The gauge allows the operator to monitor performance of the unit as well as the system in which it is installed. Rotary positive displacement blowers eat bourdon tube type pressure gauges for lunch. The pulses generated by the impellers passing the inlet and discharge connections cause fatigue failure of the pressure gauge operating mechanism. Liquid filled gauges with pulsation snubbers are high recommended. A gauge cock should be provided so that the pressure gauge is exposed to the pulses only during monitoring. The range should be selected such that the expected pressure does not exceed two thirds of the maximum indicating pressure of the gauge. Typical ranges used for blower applications are 0-15 psig and 0-30 psig for pressure applications. Vacuum gauges are typically rated from 30-O”Hgg. Most pressure gauges are rated for 150°F, therefore, the gauge must be protected from excessive temperature. Since many blower applications exceed 150°F, copper tubing should be utilized between the process and pressure gauge. Dependent on expected ambient temperature conditions and the length of tubing used, the gas can be cooled to within allowable limits. If the temperature is high and the distance between the pressure tap and the gauge is short, a coil can be Used to reduce the temperature to 150°F or less. Gauges should be remotely mounted if possible to reduce the damage done by vibration of the equipment. Sometimes remote mounting is not practical so be sure to utilize a liquid filled gauge to dampen the vibration.
Filtration of the inlet gas stream is important for protection of the blower. Most manufacturers prefer the Use of 10-micron filter elements to reduce particulates. Air filters are available for atmospheric air. Inline filters are available for closed loop applications, both pressure and vacuum. If atmospheric air is required, the filters are available with or without weather enclosures and with optional silencing capability. The silencing option can help reduce the amount of noise that “leaks” back through the filter element. Filter element material that is typically available is paper, polyester and wire. The paper offers the greatest efficiency but is difficult to clean and reuse. Polyester usually provides about 90% efficiency at 10 microns but can be washable and reused a number of times before replacement. Wire is used to prevent larger particles from entering the blower. Wire is best used in installations where the primary objective is to reduce the number of air filter changes and to reuse the filter element for several years. Dipping it in an oil bath or applying light oil can enhance the wire elements efficiency. This also helps prevent rust. The paper is the least expensive, polyester is in the middle of the pack and wire is the most expensive. Pre-filter elements made of foam are available on some brands. Utilization of the pre-filter can reduce the number of changes required of the filter element. It may be removed and washed and placed back into service. Most filters can be fitted with a device that can visually indicate when the filter should be changed. A differential pressure gauge can be used to advise maintenance personnel when the filter should be replaced or cleaned.
An absorptive silencer is typically a tube with packing. It has a low-pressure drop and is primarily used to reduce high frequency emission. It can be used on higher speed blowers (greater than 4000 RPM) but is not as effective on lower speed units. They can be effective on the discharge stack of a vacuum pump after a chamber or chamber-absorptive type silencer.
A chamber type silencer can be used on the inlet or discharge side of a blower and is best Used below the blower transition speed. The blower transition speed is determined by calculating the gear tip speed. If the gear tip speed is below 3300 fpm (feet per minute) for the inlet and 2700 fpm for the discharge, a chamber type silencer can be used with good results. Gear tip speed can be calculated by Gear Diameter in inches times 0.262 times RPM (revolutions per minute). Silencers reduce the air stream noise by Up to 30 dBA, however, the designer must remember that this does not mean the overall noise level will be reduced by this amount. Noise will emulate from the silencer shell. Additional silencing can be achieved by “lagging” the shell. Typically lagging is an acoustical insulation material that also is temperature resistant. A thin gauge steel shell typically covers the insulation
A chamber-absorptive type silencer combines chambers and absorptive material for noise attenuation at speed greater than transition speed. The absorptive mater is usually located at the inlet and outlet connections. The absorptive material helps reduce the higher frequency noise. There are typically two grades of chamber absorptive silencers, industrial grade and residential grade. Lagging is available on these units.
Silencers help reduce the magnitude of the pressure pulses produced inside the blower. These are useful to protect equipment downstream of the blower. These pulses can damage some types of heat exchangers. They are available in several configurations, straight through, side connection and top discharge, side to side (not as commonly used due to the pulse loading and frequency of weld failure).
Check valves should be used on systems where multiple units are operated in parallel or where there is a risk of material (liquid or solids) entering the blower from the process. A low-pressure drop (below 2″ H20) check valve should be utilized to reduce the loading on the blower. It must provide a positive seal when the blower is not operating. If elastomers are used for sealing, make sure that they are compatible with the gas handled and are rated for the temperature the gas passing through. Follow the manufacturers instruction regarding installation in a vertical or horizontal run of pipe. Some check valves required a different spring if mounted in the vertical position. Check valves should not be mounted within 2 pipe diameters of an elbow. Wafer type check valves mount between two flanges. Inline check valves are available to mount as a nipple.
Check valves should be mounted at least 2 pipe diameters away from a blower inlet or discharge. The pulses generated by the blower have a tendency to fatigue the springs of a flapper type valve causing failure. If a check valve is located on the inlet, be sure a relief valve is placed between it and the blower connection.
Lubricants are an important component of any blower system. Most blowers require a high quality non-detergent, non-foaming rust inhibiting oil. Depending on the operating temperature of a unit an ISO 150 or 220-weight oil should be used. Special oil formulations are available from the blower manufacturers to enhance the life of the lubricant. Synthetic oils can be used that cover a wide range of operating parameters. Synthetic oils are more expensive then mineral based oils but typically offer longer life and fewer changes. Most manufactures recommend oil changes every 500 hours for mineral based oils and may allow 6000 hours for synthetic oils. Oil should be changed based on its conditions and life can vary dramatically based on operating conditions such as temperature, pressure and process characteristics.
Oil levels should be checked weekly if possible. By no means should a continuous duty unit be checked less than once per month. Some units have grease-lubricated bearings. Be sure to use high quality grease. Most require No. 2 bearing grease with extreme pressure additives. The Sutorbilt California Series and Roots URAI series have zerk fittings with grease vents. The bearing covers should be filled such that grease can be seen coming out the grease vents. These units present a small housekeeping problem because of the build up of grease underneath the vents. Be sure to keep the grease vents clear and free flowing.