Continuous vs Periodic Monitoring

There are two types of condition monitoring: continuous and periodic. Continuous monitoring, as its name implies, examines measurements taken on a continuous basis. Periodic monitoring is based on measurements taken at regular time intervals.

With the advent of computerized real-time systems, the distinction between continuous and periodic condition monitoring must be modified. Though technically periodic, a scanning system operating fast enough to protect against catastrophic failure is considered continuous. Most people consider one second or faster scan rates as continuous. A scan rate of one second is defined as monitoring each point once each second.

The argument that an analog system dedicated to each measurement parameter provides better protection than a scanning system simply is not true. An analog system has inherent time delays that result in finite

Figure 8-37. Typical axial proximity probe installation. (Courtesy of Turtjocare, A Division of Demag Délavai Turbomachinery Corp. Houston facility)

response time to a step increase. Some users have demanded computer systems with exceptionally high scan rates to duplicate the response they thought they had with analog systems. In actuality, a relatively low scan rate of ! -2 seconds is normally sufficient to duplicate the performance of analog condition monitoring systems. Periodic monitoring once meant taking measurements manually at intervals that varied from one week to three or six months. Periodic monitoring can now be redefined as measurements taken at intervals that are too long to provide protection against a sudden failure. In general, measurements taken at intervals longer than 5 seconds can be considered periodic.

What are the advantages and limitations of continuous and periodic monitoring? Continuous monitoring requires a relatively large initial expenditure. But once installed, cost of operation is quite low. Periodic monitoring has a low initial cost, but is manpower intensive and therefore has a relatively high continuing cost.

Continuous Monitoring

Continuous monitoring is necessary on critical machines where problems can develop rapidly and have severe financial consequences. Typical machines in this category are unspared process compressors. Remotely located machinery such as pipeline gas compressors also require continuous monitoring. Also, continuous monitoring may be dictated by safety considerations. Even though the cost of a failure is small, machines should be continuously monitored if a failure will result in hazards to personnel. Figure 8-38 depicts a typical continuous monitoring system.

Periodic Monitoring

Periodic monitoring is typically applied to less critical machinery where advance warning of deteriorating conditions will show a positive return on investment. Another form of periodic monitoring is the detailed analysis of dynamic data from critical machines. Signals from sensors installed for continuous monitoring of overall vibration level, together with additional temporary sensors, are spectrum analyzed and compared against previously accumulated data. This is sometimes referred to as signature analysis. In some cases, changes in vibration signatures will provide earlier warning of deterioration than changes in overall vibration level.

Almost all compressor operations will be more satisfactory if a control system is included. Hence, it should always be considered during the original planning phase. Manual operation will result in an operating pattern that differs considerably from a controlled one. Proper evaluation of pat terns is required before the need for a control system can be established.

Figure 8-39 illustrates the operating characteristics for a simulated manual and automatic control. In any specific case, the process engineer and the compressor design engineer must cooperate in establishing some of the needs for and capabilities of the compressor.

The design of a control system is a job of considerable magnitude and a general understanding of the engineering approach is necessary to prevent underestimation of the work to be done.

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Responses

  • haris
    What isthe difference between continuous and periodic monitoring?
    3 years ago

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