Measurement Techniques for Reciprocating Compressors

  • February 07, 2000
  • News
Prepared for presentation at the 1997 Gas Machinery Conference
Austin, Texas

Using standard vibration measurement methods on reciprocating machinery, to determine its running condition, has demonstrated itself to be unreliable! For years we, as an industry, have applied the techniques which have been mastered for measuring and understanding rotating machinery, to reciprocating machinery, only to produce confusing results. There was nothing wrong with the instruments we used or the methods we used to obtain the measurements. We were trying to do something that we should not have been doing.

This discussion provides a review of present compressor monitoring systems and introduces two new measurement techniques, Impact and Triangulated Rod Drop, that were developed especially for reciprocating compressors. Examples of measurement techniques and some actual data taken on a large integral gas fired engine and compressor, typical of what is used for gas production and gas transmission, will be reviewed. While vibration is still a concern, mechanical looseness and wear are of greater concern because of the damage potential that can result when these large machines start to develop mechanical problems. These two new measurements monitor mechanical looseness and wear on compressor cylinders with a high degree of dependability.

What to Monitor
Deciding what parameters to monitor on a compressor can be a difficult decision. There are good sound choices today. If your company has experience with monitoring systems, the choice/benefits are a little clearer. If not, you will have to choose which parameters to monitor based on their merit and your budget. Far too many compressors only have vibration switches installed on them for protection. While they provide a trip in an emergency, they do nothing towards assessing the running condition of the machinery.

The table below, Typical Measurements, provides a summary of monitoring choices. It identifies the type of measurement and the part of the compressor or other machinery that is applicable.
Typical Measurements
Impact Compressor Cylinders
Rod Drop Compressor Rods
Velocity Casing Vibration
Fin Fans
Auxiliary Equipment
Impact or Velocity Engine Main Bearings
Temperature Suction and Discharge Gas
Bearing Temperatures
Electric Motor Windings

When designing a monitoring system, it is important to have the end result clearly defined. For example, is the intent to have a safety shutdown monitoring system or is the purpose to provide operating condition data that operators can use to make run/don't run decisions? Also, is the operating condition data going to be used to help mechanical equipment engineers assess plant equipment availability? The latter might require monitoring system interfaces to other plant instruments or control systems.

Impact Monitoring
Impact monitoring has proven itself to be a sound measurement for detecting mechanical looseness on compressor cylinders at early stages of development. An impact sensor is placed on the cross-head or extension piece to make this measurement. Mechanical conditions such as loose rod nuts, loose bolts, excessive slipper clearance, worn pins and liquid in the process can be detected using impact monitoring.

There is a significant difference between a vibration signal and an impact signal. A vibration signal is considered to be steady state or "stationary" and conventional measurement and analysis techniques can be applied. An impact signal is non-stationary so conventional measurement and analysis techniques cannot be used. An example of a vibration signal is shown in Figure 2 and an example of an impact signal is shown in Figure 3. Both examples are displays of amplitude vs. time. Notice the impact signal has short duration "peaks"(referred to later as events) that appear occasionally. The measurement in the monitoring channel for impact is peak g's (acceleration) in order to measure the high amplitude, short duration events. If we were going to measure vibration, we would use a velocity measurement. In this application, we are not concerned about vibration. The very concept of reciprocating machinery will produce vibration but it is normally not serious as related to the health of the machine. But looseness can be!

The impact sensor is placed normal to the direction of motion of the rod. (If we wanted to measure vibration, we would not place it in that direction). It is typically installed on top of the cross-head or extension piece where it will be out of the way of routine work or inspection. We are taking advantage of the mechanical transfer of energy, caused by impacts resulting from looseness, through the machine case.

Measuring peak g's to detect mechanical looseness is a valid technique. But some interpretation of the data is required in order to have a reliable monitoring system. A measurement technique was developed that qualifies the impacting events in order to avoid nuisance alarms. The measurement is made with a special detection circuit that was designed for this application. The qualification of the measurement is based on both amplitude and whether or not the events are singular events or repeated events. Upon detection, the events are counted. If the designated count is not reached, the counter resets and waits to be activated again. Two alarm levels, alert and danger, (or high and high high) are provided. Separate level and count criteria are used for alert and danger alarms which further insures credible notification of machine problems.

Rod Drop Monitoring
Rod Drop has been a popular measurement on reciprocating compressors. It is intended to measure the wear on rider bands, rings and seals. The measurement is made with a non-contacting displacement eddy probe and displacement measuring channel cards that were originally developed to measure radial vibration and axial position on high speed rotating machinery. The parallels to monitoring rods on reciprocating compressors are; the radial vibration measurement will show rod runout while the axial position measurement will show rod drop. Rod runout is the ac part of the signal and rod drop is the dc part of the signal. A diagram of the eddy probe and driver is shown in Figure 4 along with a typical plot of its output.

The output voltage increases when the probe gap increases which makes it an ideal sensor for compressor rod measurements. Two potential difficulties can make these measurements less sensitive than what is desired in order to know the running condition of the rod. The first is simply that there is too much mechanical runout during one cycle which makes measuring changes of 3 or even 5 Mils very difficult. Another is usually the result of repair work. If a rod is plated, the surface below the plating is usually irregular. Since the eddy probe penetrates below the surface, it electrically sees this irregular surface. This is called electrical runout and can mask any attempts to make accurate displacement measurements. Figure 5 shows examples of the displacement traces (almost 2 cycles) measured on two rods on a compressor. Both show peak to peak values that are high and will make it difficult to see any gradual trends caused by wear.

Standard rod drop and rod runout measurements are made continuously during each complete cycle of operation. In order to overcome the effects of mechanical and electrical runout, a displacement measurement must be "gated" on and off. By synchronizing this with a tachometer signal on the crank flywheel, the point on the rod where the measurement is made can be designated by the monitoring channel. This concept can be taken a step further by offering "triangulated measurement". In this mode, two readings are taken and a third measurement point is projected.

The user can elect this third point to represent the rider band area. In this case, the monitor indicates the displacement reading as if the sensor were inside the compressor cylinder and capable of measuring rider band wear directly. The monitoring channel offers other measurement options and default conditions in case the tachometer signal is lost during operation.

Auxiliary Equipment Monitoring
When planning a monitoring system for a compressor, it is easy to add a few channels and gather some operating data on auxiliary equipment. For example, if your compressor has a motor drive, add measurement points at the coupled end and the outboard end of the motor. These should be in the horizontal direction.

If you are operating integral gas fired engine compressors or separates, there are several points that should be monitored. Among these are cooling water fans, turbochargers, water pumps, and oil pumps. By mounting a sensor inside the engine on main bearing supports, detonation and mechanical looseness can be detected.

Generally velocity vibration is the best type of measurement for auxiliary equipment. The sensor is usually an accelerometer but the signal is converted to velocity in the monitoring channel. Most monitoring systems allow the user the ability to select the bandwidth of the measurement frequency range. This allows a monitoring system to be tailored to the type of machinery and block characteristics that are known to exist but are not critical. The unwanted characteristics are not included in the measurement and can not be the basis for alarms.

The following table provides a summary of the types of measurements that are typically made on machinery and the types of sensors that are used. Notice that the type of sensor does not always determine the type of measurement. With the high reliability of electronic circuits and components available today, additional processing of a sensors signal can produce the best type of measurement (and monitor channel reading) for the application.

Measuring suction and discharge temperatures can reveal important cylinder and valve operating information. If your Control Processor is not already monitoring these temperatures, they can be added to the vibration and impact monitoring system. Most temperature monitoring systems will allow you to monitor absolute values, average temperatures, and differential temperatures, among a selected group.
Summary of Measurement Types
Type of Sensor Type of Measurement
Accelerometer Acceleration, g's
Velocity, ips
Displacement, Mils p-p
Impact Sensor Impact, g's
Velocity Pickup Velocity, ips
Displacement, Mils p-p
Eddy Probe & Driver Displacement, Mils p-p

Monitoring System Requirements
Compressor monitoring systems provide continuous monitoring of current operating conditions. Because of this, they can be used for safety shutdown, if the process will allow a machine to be tripped off line. An alert alarm always precedes a trip alarm. Another important form of the operating data is the trend. While the current absolute reading is informative to a trained analyst with experience on a particular machine, a trend tells an easy-to-understand story of the changes in the machine over time. Some systems offer internal trending of measured parameters. Other systems require trending of data be done externally by a PLC, DCS or PC-based system.

Most monitoring systems are designed to meet minimum expectations of users in terms of features and functions. One industry standard was introduced several years ago by the American Petroleum Institute (API). API Standard 670, 3rd Edition, is the current document that outlines capabilities for monitoring systems, including transducers, that provide interchangability, reliability, and standard functions. All of which make it easier for the user.

Among some of the features and functions of monitoring systems designed to meet API 670 are; the ability to interchange component parts of a system without requiring recalibration, standardized sensitivities, dual alarm set points (high and high high), alarm first out indication, system OK monitoring, independent channel integrity, and recommended transducer arrangements for various types of machinery. While API 670 is not a mandatory requirement for reciprocating compressors, the general guidelines for monitoring systems is useful when specifying systems.

In summary, a minimum compressor monitoring system should include the following:
a. Suction and discharge valve temperature
b. Impact on each compressor cylinder
c. Casing vibration (velocity)
If the budget permits, add:
d. Rod drop on each compressor rod
e. Driver vibration (the type of driver will cause this to vary)
f. Motor winding temperature
For gas fired engines, add:
g. Turbocharger vibration
h. Main bearing vibration
i. Water pump vibration
j. Cooling fan vibration
This article is provided by Hardy Instruments.

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