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Reciprocating Compressor Clearance Term

Reciprocating Compressor Clearance – Simulated vs. Analyzer Measured

One of the most asked questions when it comes to utilizing a reciprocating performance analyzer is where does the cylinder end clearance value come from? This becomes more of a contentious issue when dealing with simulation programs such as Ariel, Superior, Gemini, DRSize and with remote performance monitoring software simulations and/or services.

The confusion comes from what the clearance term affects as far as the calculated results from a simulation program versus what the reciprocating performance analyzer software calculates as the C or Clearance term. In a simulation of compressor horsepower and capacity, the C or Clearance term is one of the more sensitive values in determining the compressor end VE or volumetric efficiency. From that value cylinder horsepower, capacity and many other factors are calculated to simulate compressor performance.

Breaking it down

Using the GPSA Engineering Book, equation 13-15 of volumetric efficiency VE:

GPSA Eq 13-15 VE
GPSA Equation for VE Eq 13-15

Eq. 13-15

VE – volumetric efficiency, percent
r – compression ratio, P2/P1
C – Cylinder clearance as a percent of cylinder volume
Zs Zd – compressibility factor suction, discharge
k – isentropic exponent, Cp/Cv

GPSA Cylinder.
GPSA Engineering Book. Cylinder. Fig 13-16

Clearance is the total volume remaining in the cylinder at the end of the piston stroke. This consists of the volume between the end of the piston and the cylinder head, in the valve ports and the volume in the suction valve guards and the discharge valve seats, the space between the inside diameter of the cylinder and the outside diameter of the piston from the faces of the ends back to the piston rings. Similarly, on the crank end this would be the area less the area of the piston rod. Adjustments made to cylinder volume unloaders, valve spacers and pocket unloaders all contribute to the cylinder clearance.

Analyzer data capture

The reciprocating performance analyzer (ie. Recip-Trap, Windrock, Lense etc) measures the actual internal cylinder Head End (outboard) and Crank End (inboard) pressures with respect to crank angle to determine the cylinder end VE from the related Pressure-Volume diagram. Actual pressure data is used and not simulated.

Windrock PT- PV data plot
Actual PV card data from cylinder data.

From the live data capture, in this case, the VE measured for the Head End Suction is reported at 75.8% and for Head End Discharge at 47.9%.  These VE numbers are utilized to calculate the internal C or Clearance term through the 13-5 equation. The VE value is measured first and then using the current operating conditions for the pressures, temperature and gas properties, and is back calculated through the equation to determine the C or Clearance term value.

The important point here is that this value is affected by the data quality of the pressure sensor and the sensing system. Long measurement paths, obstructions in the pressure measurement path, poor test valve selection, degrading pressure sensors or incorrect pressure sensor sensitivities can affect the data quality and capture.

Also of note is the absolute affect of the measurement of the phase of the compressor to the marker input. A small change of 0.5° rotation can have a large effect on the VE calculation and subsequent data from the live pressure sample.

Simulation Clearance input

Alternately with simulation programs, the data gathering starts with the machine geometry.  Key details for the machine are input into the simulation program such as bore, stroke, rod diameters, connecting rod length and variable volume pocket information.   Depending on the simulation program, the internal geometry of the machine for the variable volume pockets, cylinder end clearances, valve areas and other factors that affect performance are already documented.  Simple measurements of the variable volume pocket positions and valve geometry data trigger values from the program for the total C or Clearance term required.  These key factors, that may or may not be accurate, account for the Clearance term that is crucial in calculating the compressor performance.  With the Clearance term and the input properties, the PV diagram is simulated.

Ariel PV-1
Ariel Simulated PV results

So what does this mean for reciprocating compressor performance?

Clearance is an important part of the reciprocating compressors performance.  The primary difference being that as a performance analyzer sees it, the pressure in the cylinder with respect to the rotation of the crankshaft is measured.  That does not change any of the facts derived from this data where the Clearance term is calculated from that data.  The analyzer does not care what input value you put in for Clearance, the value is going to be calculated from the live data captured and can not affect the pressure measurements.

The sensitivity of the C or Clearance term for the simulation programs is key.  Many times I have been called to verify the performance of machinery based on simulated results and what is actually being performed by the machine.  A very high percentage of the problem comes back to the simulation not having accurate information on the internal pressures and clearances from the geometry of the machine.  Differences in variable volume pocket position, valve spacers or valve area and changes in geometry of the machine from repairs account for value for C or Clearance different from the simulation value versus the measured value from the analyzer data.

A key to using the live data capture is to fine tune your simulation calculations to match the live data.   With this adjustment to internal pressures, Clearance and measured losses you can make the simulation data as close to live as possible.   Future simulations using that data will more accurately account for any changes you want to attempt.  Changes in speed, loading conditions or load steps can all be more accurately simulated and with confidence you will be operating the unit safely.

Jason Hoffman, C.E.T.

Jason has over 25 years of experience in compressor performance analysis using Recip-Trap and Windrock analyzers to provide these services as well as multitudes of simulation programs to optimize performance.

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