Page 234 - Compression Machinery for Oil and Gas

Reciprocating Compressors Chapter 5 223

cylinder nozzle, but the point of interest is actually the compressor valve
which can be damaged by excessive high-frequency gas pulsations. But too
often orifice plates are used “just to be safe” and sized basis average flow.
It is recommended that orifice plates be sized based on instantaneous flow
so that the effective pressure drop is calculated. A low (nonconservative) esti-
mate of effective flow rate can be estimated based on the total flow divided by
the percentage of time the valve is actually open (70% on inlet and 50% on
discharge is typical). The true pressure drop will be even higher that this
calculation, but this is a useful and simple to calculate method of estimating
cylinder nozzle orifice plate pressure drop. For further discussion of this topic
refer to Ref. [21].

Nozzle Pulsation
As noted previously the column of gas on the outside of the valve all the way
back to the junction of the pulsation bottle nozzle-pulsation bottle goes from
being stationary while the valve is closed and has to accelerate to maximum
velocity when the valve opens and then slows back to zero and the piston
reaches the end of the stroke. This acceleration and deceleration of the gas
causes pressure pulsations. The severity of the pressure pulsation is related
to the velocity head of the column of gas, so it is related to the gas velocity,
the gas acceleration and the mass of the gas column. This effect is the dominant
cause of pressure pulsations in the inlet and discharge nozzle at 1 and 2
machine speed. The 1 and 2 frequency are nonresonant. In the paper by Hin-
chliff and Bratek (Ref. [7], a discussion of the loads used to rate reciprocating
compressors, GMRC 2014) a severity index was proposed to indicate the sever-
ity of pressure pulsation “severity index¼(specific power gas velocity
S.G. rpm column length)/60,000” Where specific power is the cylinder
IHP/cylinder power rating, this is just a measure of how heavily loaded the cyl-
inder, high-loaded cylinders are problematic than lightly loaded. Gas velocity is
the average piston speed (piston area/cylinder nozzle passageway area) fpm,
S.G. is the gas specific gravity so is MW/28, rpm is the compressor rotation
speed, column length is the linear distance along the gas passageway from
the compressor valve to the nozzle-pulsation bottle volume junction in feet.
The paper proposed that values less than 30 resulted in insignificant pulsation,
whereas greater 60 resulted in significant levels of pulsation. Significant pulsa-
tion levels may exceed the API 618 limit at 1 and 2 and may cause increased
rod load and horsepower over that which is calculated in the OEM’s compressor
performance program. Note that cylinder orifice plates will do little or nothing
to reduce 1 and 2 pulsations, indeed as they simply apply a resistance they
will normally make the situation worse. Orifice plates are useful to reduce res-
onant pulsations which are typically at 4 or 6 or greater harmonics. 1 and
2 pulsations can only be reduced by reducing the severity index, which means
reducing the velocity head of the gas. (Use a weld neck flange nozzle to