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Wide-open vs Working Monitors

Monitors are a commonly used form of overpressure protection in large gas distribution applications such as city gate stations, district stations, and large industrial meter sets. The vast majority of monitors in use today are wide-open monitors; however, it is important that working monitors be reconsidered due to their advantages on increasingly important factors such as noise and assurance of safe pressure delivery.

 

Advantages of a Wide-open Monitor

  1. Lower Complexity - Having one less pilot makes establishing setpoints easier and quicker and makes the system simpler to understand. Also, working monitors are uncommon at some companies, making them seem more complex than they really are. Companies who have instead standardized on working monitors achieve a similar level of ease and familiarity.
  2. Shorter run length - Regulators can be butted together whereas working monitors require volume between the regulators to achieve satisfactory performance 

 

Advantages of a Working Monitor

  1. Assurance that the auxiliary main valve is functional - Working monitors are the only form of overpressure protection that provides live feedback that it is responsive and fully capable of taking over if necessary. A high-pressure alarm using SCADA or a token relief alerts when the upstream regulator is no longer functional. The monitor pilot is still an unknown but a filter in the pilot supply line reduces much of the risk. Some companies even install a 2nd monitor pilot for further assurance.
  2. Reduced Noise - Noise is generated at a regulating station primarily by the flow rate and pressure drop. Splitting the pressure drop lowers a significant noise producing factor. In addition, a lower pressure drop ratio, as defined by (P1-P2)/P1, generates a few dBa lower noise in the regulator and can make noise attenuating trim more effective. To further reduce noise, the optimal intermediate setpoint can be calculated (just remember to verify the flow requirements are still met).
  3. Main Valve gets exercised - Boot-style products with thick boots can take a set if they are held in one position, causing delayed reaction. This occurs in upstream wide-open monitor setups if the monitor regulator bleed is piped downstream. This issue only occurs with boot-style regulators with thick boots and a common solution is to pipe the monitor regulator’s bleed to intermediate which causes the monitor’s boot to move with changing flowrate. 
  4. Lockup at downstream worker’s lockup - Some wide-open monitor configurations lockup at the monitor regulator’s lockup instead of at the worker’s lockup which prevents the use of some types of alarms. Working monitors lockup at the worker’s lockup.