MICHAEL W. GILLIS P. ENG
GROUP LEADER, MEASUREMENT AND PROCESS CONTROL ENGINEERING
In the transportation of natural gas from wellhead to consumer, service conditions will be encountered which may cause the gas and equipment used to regulate and control pressure to experience freezing conditions. Ignoring these design considerations will result in reduced facility reliability and increased operating and maintenance costs. Regulator or control valve freeze off at pressure reducing stations, resulting from the cooling effect of pressure reduction, can cause serious operational and safety problems. The freeze off can result in the regulator becoming inoperable causing a potential loss of pressure control or even result in the loss of service to downstream customers. Clearly, regulator freeze off is an operating condition a natural gas transportation pipeline or distribution utility wants to avoid. Awareness of the causes and effects of freezing conditions and methods of avoiding these conditions will result in more reliable, cost effective facilities and reduced risk of failures and safety hazards
The Effects of Regulator Freeze Off
The effects of regulator freeze off and its associated conditions are numerous and can be.
divided into two categories. Those resulting from a freeze off within the gas stream, either in the main body of the regulator or within pilot/controller piping, and the effects on the external regulator/valve body, station piping and foundations. The freeze off can occur in the small diameter instrumentation and control piping or the orifice within the pilot rendering the regulator inoperable. Depending on the regulator or control valve type and it’s action, the regulator failing to respond to a downstream condition will either overpressure or fail to adequately supply the downstream demand. In the case of a frozen orifice within a controller, the inability to vent the actuator to oppose spring action would result in a fail open or fail closed situation, again potentially losing service to the downstream customer or over pressuring downstream piping.
For external effects, a substantial volume of ice may build up on the outside of a regulator, control valve and related piping making the facility inaccessible for operation, maintenance and repairs. There may be sufficient cooling at the regulator to reduce the temperature of the regulator and surrounding pipe and flanges below it’s design limits, increasing the risk of a brittle failure. The amount of cooling occurring at a regulator can potentially cause frost heaving of downstream piping or foundations resulting in permanent damage to the structure, housing the regulators. Piping stress may be increased due to dislocation of piping that is displaced by ice formation.
Table 1 - Summary of the conditions and effects of regulator freeze off;
|Conditions |Effects | |Freeze off in pilot or |Potential loss of service or over pressure| |controller orifice |downstream | |Ice plug in regulator or |Restriction causing excessive pressure | |control valve body |drop or loss of service | |Ice build up on exterior of |Inaccessible for maintenance, inoperable | |regulator | | |Excessive cooling |Frost heaving of piping or foundation, | | |piping design limits |
A Review of Gas Quality
It would clearly be a situation of focusing on the symptom rather than the problem to ignore a review and investigation of gas quality specifications and monitoring when dealing with design and operation of natural gas regulating facilities and regulator freeze protection. A regulator is much more susceptible to freeze off due to the Joule- Thompson...