Section 4 - Part Replacement For models KBZ:U

Variable Volume Clearance Pocket (VVCP)

A VVCP changes the clearance volume of the head end of a cylinder. The amount of clearance
depends on the position of the clearance pocket piston. Users turn the piston/stem assembly
counterclockwise to increase clearance, and clockwise to decrease it. In the event that gas begins to
leak from the VVCP vent, replace the V-packing or seal within the VVCP.

VVCP Removal

WARNING: To reduce risk of injury or death, only trained personnel who have read
and thoroughly understand the appropriate Ariel Technical Manual and Packager
information should attempt any maintenance.
Prior to performing any maintenance activities:
 Always wear appropriate personal protection equipment and follow your company
safety and operational procedures.
 Never tighten or loosen any fastener while the unit is operating or pressurized.
 Verify neither driver nor compressor can turn during maintenance.
 Follow packager instructions to completely isolate and vent the system; call the
packager for assistance.
 Verify each compressor cylinder is relieved to atmospheric pressure. To relieve
residual cylinder bore pressure after blowdown, open the “Kiene” valves on the
indicator ports, if so equipped.
WARNING: RELEASED GAS MAY BE POISONOUS AND/OR FLAMMABLE.

Before resuming operation, purge the entire system with the gas being compressed to
avoid a potentially explosive gas/air mixture in the system. A gas/air mixture under
pressure can explode and cause severe injury or death.

1. After venting the cylinder, loosen

the locking handle so the
1. Cylinder
adjustment handle can turn freely.
Turn the adjustment handle. 2. Eyebolt
3. VVCP Vent
4. Locking Handle
5. Adjustment
Handle
6. Flange Bolts
7. Bellows Thread
Protector
FIGURE 4-2
Typical VVCP

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CAUTION: A hard to turn adjustment handle or piston stem may indicate pressurized
gas trapped within the VVCP. See VVCP Disassembly caution below.

2. Tighten the locking handle.

3. Disconnect the VVCP vent.
4. Remove plastic plug from eyebolt hole and thread an eyebolt into the eyebolt hole.
5. Use the Ariel supplied eyebolt to support the weight of the VVCP.
6. Remove all flange bolts and slide the VVCP out from the cylinder. The Ariel Performance Program
contains approximate VVCP weights.
If the VVCP does not slide out easily, loosen the valve cap bolts. If it still does not slide out easily:
a. Remove head end valves.
b. Clean the area between the VVCP head and the cylinder.
c. Loosen discharge bottle supports.
7. Discard the head gasket and transport the VVCP to a suitable work area for disassembly.

VVCP Disassembly
The VVCP piston ring is not gas-tight to allow a nearly balanced gas pressure, which eases VVCP
adjustment with the cylinder pressurized. Gas pressure behind the VVCP piston normally vents when
the cylinder vents. Process debris or rust around the piston ring can form a seal that traps gas within the
VVCP assembly, sometimes at substantial pressure.

CAUTION: Trapped gas pressure can present a personal safety hazard when servicing
the VVCP. Work in a well-ventilated, non-sparking area, particularly with sour gas
applications. Do not breathe gas emission from VVCP when venting trapped gas.

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Section 4 - Part Replacement For models KBZ:U

1. Locknut 6. Grease Fitting 12. Piston 18. Compression Spring

2. Adjustment Handle 7. Adapter 13. Piston Ring 19. Backup Ring
3. Adjustment Handle 8. Match Mark 14. O-Ring (see Note 2) 20. V-Packing or Seal
Lip 9. Piston Stem 15. Retaining Plate 21. Seal
4. Bellows Thread 10. Socket Head Bolt 16. Snap Ring 22. Seal Carrier
Protector 11. Head 17. Spring Retainer 23. O-Ring
5. Locking Handle
FIGURE 4-3 VVCP Disassembly/Assembly

1. Place the VVCP on a table, adapter up.

2. The bellows thread protector grips a lip on the adjustment handle. Use pliers to pull the bellows from
the adjustment handle lip. Do not tear the bellows.
3. Remove locknut.
4. Remove adjustment handle. The adjustment handle is a tapered fit onto the piston stem; use a soft
hammer or puller to break the fit.
5. Thread the locking handle off of the piston stem. Leave the bellows thread protector attached to the
locking handle.
6. Depending on size, a VVCP may have two to four socket head bolts. Use a marker to draw match
marks on the side of the VVCP where the adapter and head join at each socket head bolt location.
Loosen the socket head bolts evenly and incrementally to release any potentially high-pressure gas.
Do not breathe gas emission from VVCP.
7. Remove socket head bolts and turn the adapter counterclockwise to thread it off of the piston stem.
Place adapter flange up next to the head.

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8. Remove the piston stem and piston assembly from the head. The piston stem and piston are
permanently joined; do not attempt to disassemble them.
9. Remove the piston ring from the piston.
10. Depending on VVCP size, Ariel machines a groove for the o-ring into either the face of the head or
the base of the adapter. Remove and discard the o-ring.

NOTE: Steps 11-13 apply only to VVCP's with a V-packing seal.

11. Some adapters use a bolted retaining plate while others
use a snap ring and spring retainer:
a. Retaining Plate - Remove retaining plate bolts and
retaining plate.
b. Snap Ring and Spring Retainer
 Thread a nut onto a threaded rod that is a few
inches longer than the adapter height.
 Slide a washer wider than the main bore of the
adapter down to the nut.
 Insert the threaded rod end with no nut into the
main bore of the adapter until it protrudes from the
base of the adapter.
1. Washer 5. Backup Ring
 Slide a washer wide enough to overlap spring
retainer but less wide than the snap ring onto the 2. Snap Ring 6. Spring Retainer
protruding threaded rod. 3. Threaded Rod 7. Compression
Thread a nut onto the threaded rod protruding from Spring
 4. Nut
the base. Tighten nut until compression spring 8. V-Packing or
compresses enough to separate spring retainer Seal
from snap ring. FIGURE 4-4 Spring Compression for
 Use snap ring pliers to remove snap ring. Snap Ring Removal

 Remove nut from threaded rod. Remove threaded

rod and washers from adapter main bore.
12. Remove spring retainer, compression spring, and backup ring from the adapter counterbore.
13. Remove V-packing or seal from adapter counterbore. Find a strong, foot-long metal 3/16 inch rod or
pipe and bend about .75 inch at 90°. Insert the rod into the adapter counterbore and slide the .75 inch
beneath the V-packing or seal. Pull up on the rod or pipe to pull the V-packing or seal from the
adapter counterbore.

14. Using an appropriate puller tool, remove the first seal from the seal carrier bore. Be careful not to
damage the seal housing surfaces.
15. Remove the four socket head bolts, seal carrier, and second seal from the adapter bore. Remove
the O-ring from the seal carrier.

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 Section 4 - Part Replacement For models KBZ:U

VVCP Reassembly
Clean all VVCP parts of all debris, rust, etc. Replace worn or damaged parts.
1. With flange up, place the head on a table. With flange up, place the adapter beside the head.
2. Depending on VVCP size, Ariel machines a groove for the O-ring into either the face of the head or
the base of adapter. Oil and insert a new O-ring into O-ring groove.
3. Install the piston ring onto the piston.
4. Oil the head bore, then insert stem and piston assembly into the head cavity.

NOTE: Steps 5-9 apply only to VVCP's with a V-packing seal.

5. Oil the adapter counterbore and insert V-packing or seal. The V-packing or seal consists of five
partitions; insert V-packing or seal into the counterbore with the thickest partition down and the
thinnest partition up.
NOTE: When properly installed, a V-packing gives resistance when the stem is inserted. If
there is no resistance, remove the stem and use a solid cylinder of wood or metal to tap
the V-packing or seal two or three times into the counterbore to ensure proper seating.
6. Insert backup ring into adapter counterbore.
7. Insert compression spring into adapter counterbore.
8. Place spring retainer on top of the compression spring.
9. Some adapters use a bolted retaining plate while others use a snap ring and spring retainer:
a. Retaining Plate - Place the retaining plate on top of the spring retainer and press down to
compress the compression spring. Install retaining plate bolts through the retaining plate and into
the adapter. Tighten the retaining plate bolts hand tight.
b. Snap Ring and Spring Retainer
 Compress the compression spring (see FIGURE 4-4).
 Use snap ring pliers to install the snap ring above the spring retainer.
 Remove the nut from the threaded rod, then remove the threaded rod and washers from the
main bore of the adapter.

NOTE: Steps 10-12 apply only to VVCP's with a spring energized seal.
10. Lightly oil and install a new seal in the VVCP adapter bore with the spring facing up. Lightly oil and
install a new O-ring on the seal carrier.
11. Install the seal carrier in the VVCP adapter, tightening the four socket-head bolts to the torque listed
in Appendix A.
12. Lightly oil and install a new seal in the seal carrier with the spring facing up.
13. Grease piston stem threads and install adapter onto piston stem. Install adapter as far as possible on
the piston stem, then turn the adapter counter-clockwise to align the socket head bolt match marks
on the adapter flange with the match marks on the head.
14. Install the socket head bolts. Tighten socket head bolts to the torque listed in Appendix A.

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15. Install locking handle onto the piston stem, but do not lock it. Assemble the locking handle with the
bellows thread protector facing up.
16. Slide the adjustment handle onto the piston stem and use a soft hammer to ensure full engagement
of the tapered fit.
17. Use the adjustment handle to turn the piston stem counter-clockwise to tighten the seal between the
adapter and head, then lock the locking handle.
18. Install the locknut onto the piston stem and tighten it to prevent the adjustment handle from
loosening.
19. Slide thread protector bellows onto the adjustment handle lip and verify that it grips securely.
20. Apply 3-4 pumps of all-purpose petroleum grease with a grease pump at the grease fitting.

VVCP Installation
Install the VVCP prior to installing valves and torqueing
valve caps. Use the Ariel supplied eyebolt to support the
weight of the VVCP.
1. Apply an anti-sieze lubricant to both the new head gasket
and the cylinder seating surface. Stick the new head
gasket to the cylinder seating surface.
2. Slide the VVCP into the cylinder and align the adapter
flange bolt holes with the cylinder bolt holes.
3. Lubricate and torque flange bolts per Appendix A.
1. Cylinder 4. VVCP Vent
4. Reconnect VVCP vent. Verify piston end clearances. 2. Head Gasket 5. Flange Bolts
See Appendix B . 3. Eyebolt
5. Install valves and torque the valve caps. See FIGURE 4-5 VVCP Installation
"Compressor Valves - Removal and Installation".

VVCP Adjustment
Users may change VVCP clearance volume with the compressor running or stopped. The expected
change in compressor flow and absorbed power depends on compression ratio and properties of the
compressed gas. Consult packager instructions regarding where to set the VVCP. Also see the VVCP
data sheet in the Ariel Parts Book. To open VVCP to a desired percentage:
1. Loosen the locking handle so the adjustment handle can turn freely.
2. Find the VVCP dimension plate.
3. Subtract fully closed dimension from fully open
dimension. The Ariel Performance program also
lists these dimensions if the VVCP dimension
plate is missing. FIGURE 4-6 VVCP Dimension Plate
4. Multiply the step 3 result by the desired percentage expressed as a decimal.
5. Add the step 4 result to the fully closed dimension.
6. Turn the adjustment handle until the measurement from the base of the locking handle, when locked,
to the top of the adjustment handle equals the step 5 result.

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