Vibration compressors

Most rotating equipment includes electric motors or steam dryers that generate noise at a constant frequency. Air cooler fans are a source of noise that can be reduced by lowering the fan speed and increasing the number of blades. Pump motor noise can be reduced by including a shroud or fan cover that is accurately lined. Centrifugal compressor noise reduction can be achieved by blade design and the use of compressor pulsation noise reduction, silencers, and vibration isolation. 

Continuous-Flow Compressors Continuous-flow compressors are machines where the flow is continuous, unlike positive displacement machines where the flow is fluctuating. Continuous-flow compressors are also classified as turbomachines. These types of machines are widely used in the chemical and petroleum industiy for many services. They are also used extensively in many other industries such as the iron and steel industry, pipeKne boosters, and on offshore platforms for reinjection compressors. Continuous-flow machines are usually much smaller in size and produce much less vibration than their counterpart, positive displacement units. 

Many users consider rotaiy compressors, such as the Rootes -type blower, as turbomachines because their behavior in terms of the rotor dynamics is very close to centrifugal and axial flow machineiy. Unhke the reciprocating machines, the rotary machines do not have a veiy high vibration problem but, like the reciprocating machines, they are positive displacement machines. 

Design The economics usually work out in the favor of gas coolers when the centrifugal machines flow rate reaches about 5000 scfm. The pressure loss can Be kept to 7.0 kPa in most cases. When the ratio of A(ht lo Ajhs is 20 1, is another point to consider these plate-fin designs. Vibration is practically impossible with this design, and uses in reciprocating compressors are possible due to this. 

Nitrous gases originating from the combustion units in nitric acid plants carry small amounts of unreacted ammonia, NH3. The ammonia may react with the nitrous gas to form microscopic particles of ammonium nitrate that adhere to solid surfaces. Within a short time, there is a growing layer of ammonium nitrate salt covering the internal surface of the nitrous gas compressor (Figure 4-27). This layer can obstruct the flow passages because it tends to increase the power consumption, provoke excessive vibrations, and even present a safety hazard since ammonium nitrate explosions can occur. 

Monitoring should include the usual parameters worthy of surveillance in high-speed turbo machinery the temperature of journal bearings, vibration and axial position of the pinions, inlet and discharge temperatures combined with discharge pressure from the individual compressor stages, and various lube oil system devices. Competent manufacturers make sure all measurement locations are completely prewired on the machine and made available at predefined interfaces or in terminal boxes for connection at the plant site. 

Expander-compressor shafts are preferably designed to operate below the first lateral critical speed and torsional resonance. A flame-plated band of aluminum alloy or similarly suitable material is generally applied to the shaft in the area sensed by the vibration probes to preclude erroneous electrical runout readings. This technique has been used on hundreds of expanders, steam turbines, and other turbomachines with complete success. Unless integral with the shaft, expander wheels (disks) are often attached to the shaft on a special tapered profile, with dowel-type keys and keyways. The latter design attempts to avoid the stress concentrations occasionally associated with splines and conventional keyways. It also reduces the cost of manufacture. When used, wheels are sometimes secured to the tapered ends of the shaft by a common center stretch rod which is pre-stressed during assembly. This results in a constant preload on each wheel to ensure proper contact between wheels and shaft at the anticipated extremes of temperature and speed. 

Richards, R.F., Vance, J.M., Paquette, D.J., Zeidan, F.Y., Using a Damper Seal to Eliminate Subsynchronous Vibrations in Three Back-to-Back Compressors, Proceedings of the 24th Turbomachinery Symposium, Texas A M University, p. 59, 1995. 

Compressor and turbine seetions ean be analyzed effeetively by eombining vibration speetra with ehanges in performanee data. Major problem areas in eaeh of these eomponents ean be identified with proper monitoring and analysis. 

Compressor analysis is done by monitoring the inlet and exit pressures and temperatures, the ambient pressure, vibration at eaeh bearing and the pressure and temperature of the lubrieation system. Table 19-5 shows the effeet various parameters have on some of the major problems eneountered in a eompressor. Monitoring these parameters allows the deteetion of ... 

Retrieval by vibration levels. Inspection of data provided by this mode can be useful in determining compressor fouling, compressor or turbine blade failure, nozzle bowing, uneven combustion, and bearing problems. 

Joergensen, S. H., Transient Value Plate Vibration, Proceedings of the 1980 Purdue Compressor Technology Conference, Purdue University, West Lafayette, IN, 1978, pp. 73-79. 

Screw compressors of the dry type generate high frequency pulsations that move into the system piping and can cause acoustic vibration problems. These would be similar to the type of problems experienced in reciprocating compressor applications, except that the frequency is higher. While volume bottles will work with the reciprocator, the dry type screw compressor would require a manufacturer-supplied proprietary silencer that should take care of the problem rather nicely. 

Axial compressor blades are usually forged and milled. Precision casting has been used on occasion. The most common material used is a 12 chrome steel, in the AISI 400 series, and is also known as 400 series stainless steel. While the stator blades are occasionally shrouded, the rotor blades are free-standing. Lashing wires have been used on rotor blades, but are generally used to solve a blade vibrational stress problem. 

This chapter will cover some of the more common accessory items for compressors such as the lubrication system, gears, coupling, instrumentation, vibration monitoring, and process control. The subject is broad and far-reaching. It is hoped that, for the first-time user, this discussion will be a good introduction and, for the veteran, it may offer another perspective on the subject. 

The subject of vibration instrumentation can fill many pages, but i. only one phase of the world of compressor instrumentation. Therefore the coverage here can only be brief. 

Figure 8-38. Typical continuous monitoring system for a centrifugal compressor. (Reprinted by permission o/Sound and Vibration)...

Other sources, such as compression temperature rise, driver-induced vibration, or component problems (bows) can contribute to the machine shaking. These must be treated as they occur. As a minimum, care must be used to understand the nature of the sources to keep them from interacting with the resonant frequencies of the compressor. The best remedy to stop the excitation at the source. If this is not possible, selective tuning and proper application of damping must be used. 

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