Deactivated Stage

 
Every now and then a service changes to lower pressure ratios and requires fewer stages of compression on existing equipment.  Deactivating a stage can be accomplished with allowing blowthru or by physically deactivating a stage.  Deactivated stage methods are not to be confused with deactivating a cylinder within a multi cylinder stage (see Deactivated Cylinder below).  
 
Blowthru occurs when the compression ratio across the compressor is not high enough to accommodate compression in all the available stages.  The gas is compressed to full discharge pressure in the first several stages and physically blows thru the last stage without further compression.  This blowthru gas flow holds the suction and discharge valves open.  If the unit is close to allowing full compression on the last stage the valves can flutter during this blowthru condition.  Blowthru is acceptable if the unit will not operate at extended periods of time at this condition.
 
If the unit will operate at this lower ratio for extended periods a stage of compression can be deactivated, or taken out of service.  For shorter durations the cylinder can be deactivated by removing the suction valves from both ends of the cylinders on the deactivated stage.  For longer periods the stage can be deactivated by removing all valves, removing the piston and rod, rerouting the cylinder and packing lubrication to the frame, replacing the packing with a solid plug and rebalancing the reciprocating weights.      
 
Any changes in operating conditions warrants a review of performance to ensure proper operation within allowable limits is maintained.  Changes in  conditions requiring deactivating a stage can have impacts to rod reversal, rod loads, discharge pressure versus MAWP and potentially torsional responses.
 
These "extended periods of time" should be defined by the user based upon frequency of re-activating the stage, acceptable maintenance and the availability of manpower to change the unit configuration.  In general, blowthru can be tolerated for days to a few weeks.  Deactivating by pulling the suction valves can be tolerated for months.  Deactivating by removing the piston and rod can be used for longer durations in the terms of half a year and up.  The duration of deactivation by the blowthru and pulling suction valves are discussed due to the likelihood of higher maintenance if the durations are longer.  Blowthru conditions can cause the valves to open and close, without preferable dynamics, resulting in possible broken valves before the next loading of the stage.  Pulling the suction valves and leaving the piston and discharge valves in place can also result in failure of the discharge valves over time and will result in continued wear of the piston rings, wearbands and packing sealing rings.
 
Deactivated Cylinder
 
Deactivating a cylinder that is part of a stage is handled differently than deactivating an entire stage.  An example would be if there are two first stage cylinders and typical unloading methods are not enough, one first stage cylinder can be deactivated while the other first stage cylinder continues to compress.   If an entire cylinder within a stage needs to be deactivated, this can be done in one of two ways; Removal of the piston and rod, or non-acting the cylinder.
 
Removal of the piston and rod is the preferred method to deactivate a cylinder.  This is accomplished by removing the piston and rod, installing blank flange blinds between the cylinder and pulsation vessels (suction and discharge), installing a plug in place of the packing case, rebalancing and removing the cylinder lubrication from the deactivated cylinder.  Removing the suction valves from both ends of the cylinder can be done rather than installing flange blinds.
 
An entire single cylinder, when part of a multiple cylinder stage using a manifolded suction pulsation vessel, can be deactivated by removing the suction valves from both ends of the cylinder.  This method is not recommended by Ariel, but has been performed successfully on specific installations.  If this method is successful, it can save down time for shorter term deactivation.  In this method, the gas will be recycled from head end to crank end through the suction gas passages and will heat up.  In most cases, the heat generated will be greater than the maximum allowable average cylinder temperature of 285 F (140 C).   If this methods is used, cylinder temperature measurements must be taken at the suction gas passages.  The discharge temperature measurement location will not read the cylinder gas temperature.
 
Non-Acting Cylinders
 
A non-acting cylinder is a cylinder that is installed, but is not compressing gas.  There are several ways a cylinder would be non-acting.
  

Blowthru

All Valves Removed

Suction Valves Removed

Piston and Rod Removed

Blank Throw

Non-Acting for Startup

 
Blowthru occurs when the compression ratio across the compressor is not high enough to accommodate compression in all the available stages.  The gas is compressed to full discharge pressure in the first several stages and physically blows thru the last stage without further compression.  This blowthru gas flow holds the suction and discharge valves open.  If the unit is close to allowing full compression on the last stage the valves can flutter during this blowthru condition.  Blowthru is acceptable for short durations.  Longer durations can lead to premature valve failure in the stage with blowthru.
 
All of the cylinder valves can be removed if a stage of compression will need to be disabled for an extended period of time.  This allows a larger flow area for the gas to pass through the cylinder valve ports, reducing the pressure losses in the stage.  This also removes the valves from the wear life equation.  Removing all of the cylinder valves must be limited to deactivating an entire stage, not just one cylinder of a mutli-cylinder stage.
 
Suction valves can be removed from a cylinder to disable a stage for much shorter periods.  Some of the benefits of lower pressure loss are attained, while having a shorter downtime to remove valves.
 
Suction valves can be removed if a single cylinder of a mutli-cylinder stage needs to be disabled.  Though this method is not recommended by Ariel, it may be possible to non-act a single cylinder within a stage.  In most cases, non-acting a cylinder within a stage will result in overheating the cylinder beyond the capabilities of the non-metallic components within the cylinder.  If this method is to be used for non-acting a cylinder within a stage, the cylinder nozzle temperatures on the suction side (since suction valves are removed) must be closely monitored so as not to exceed 285 F (140 C).  This method has been known to work in some specific circumstances, while in the greater majority is not successful.  The cylinder to be non-acting must be part of a manifolded suction vessel with the other cylinder(s) active.
 
Removal of the piston and rod to disable a stage can be done when the unit will operate with a disabled stage for extended periods of time.  Removal of the piston and rod allows the wearing components to be removed and the cylinder lubrication to be shutdown for the cylinder.  This is accomplished by removing the piston and rod, installing blank flange blinds between the cylinder and pulsation vessels (suction and discharge), installing a plug in place of the packing case, rebalancing and removing the cylinder lubrication from the deactivated cylinder.  Removing the suction valves from both ends of the cylinder can be done rather than installing flange blinds.  The definition of "extended period" is determined by the end user and is a balanced of the time and equipment it takes to accomplish the removal of the piston and rod against the savings for wear parts and lubrication.
 
Removal of the piston and rod is the preferred method to non-act a cylinder within a stage.
 
Blank throws are locations on the frame that can accommodate a cylinder, but are not mounted with cylinders, ie., placing three cylinders on a four throw compressor.  There are two different balancing configurations for a blank throw, an active crosshead guide and a balancing crosshead guide.  An active crosshead guide is a guide that can accommodate the mounting of a cylinder and has standard crossheads and balance nuts installed.  A balancing crosshead guide, also referred to as a dummy guide, is a special guide with added length to accommodate larger, special, balancing crossheads.  Cylinders cannot be mounted on balancing guides.  When applying a blank throw, the reciprocating weight is not offset by gas loads from compression.  Therefore, the weight of the blank throw should be minimized by mounting the smaller of the cylinders on the opposing throw.
 
Cylinders can be non-acting for start up purposes.  When the available starting torque is not available for a fully bypassed compressor start, unloading ends of the cylinders will help reduce some of the starting torque.  This will require the use of suction valve unloaders or head end bypass devices.  Actuating suction valve unloaders on the head end can help reduce the staring torque.  In some cases, the motor inrush current must be further limited; suction valve unloaders can be applied to both the head end and crank end to further reduce the starting torque.  When both ends are non-acting for start up, a time limit of five (5) minutes should be applied for the non-acting duration.
 
  

Ariel Corporation Application Manual  
7 August  2008