Pseudo-Q Value
(Courtesy Hoerbiger Corporation
of America)
The "pseudo-Q value"
is a term used at Hoerbiger which is an indicator of the "adequacy"
of a valve. In other words, is there enough or too little valve for the
application? The
pseudo-Q value is a dimensionless value developed by Hoerbiger and Ariel
to indicate the adequacy of a compressor valve. It is defined as the average
pressure drop through the valve divided by flange pressure, expressed
as a percentage. The pseudo-Q value must be between 1% and 15%. With
high and low pseudo-Q values the compressor calculation model is not able
to accurately predict the horsepower losses, preheating of the gas during
the intake event or the volumetric efficiency (capacity).
On the suction valves the Q -
value is the pressure drop across the valve in percent of line pressure:
where = gas density
Ps
= suction pressure
V =
mean valve velocity (calculated with equivalent area)
Figure
1 - Theoretical vs. Actual VE
Ariel and Hoerbiger require q-values
between 1% and 15%.
- Pseudo-Q
value less than 1%
If Q drops below 1%, it becomes
difficult to achieve good valve dynamics, since the pressure drop which
the valve springs work against is too low and a tendency towards valve
flutter is great. This is what is commonly referred to as having “too
much valve”. This can happen when compressing low molecular weight gases (hydrogen, helium, etc.). The
solution is to lower the valve lift, lowering lift area and equivalent
area, and creating a larger pressure drop across the valve.
- Pseudo-Q
value greater than 15%
If Q is 15% or higher, the
pressure drop across the valve is too high for the cylinder pressure to
recover to line pressure at the end of the suction stroke. The valve does
not close at the end of the stroke as designed. Gas backflows through
the valve as the piston is beginning the compression stroke, which slams
the valve closed. Volumetric efficiency is also reduced since the cylinder
does not have a full charge of gas (due to a backflow out of the cylinder
bore), in addition to the high horsepower losses associated with the large
pressure drop across the valve. High q-values can occur when compressing
high molecular weight gases
(CO2,
propane, etc.). In most cases, lift cannot
be added to valve without sacrificing durability. Slower rotating speeds
will lower q-values by decreasing piston speed and gas velocity through
the valve. Selecting
a different cylinder may result in lowering the pseudo-Q value due to
valve flow areas differences.
Ariel
Corporation Application Manual
7-Sept-2001