Page 49 - Air and gas Drilling Field Guide 3rd Edition

40 CHAPTER 3 Surface Equipment

along the flow line. A low pressure gauge is placed downstream of the primary
compressors but upstream of the booster compressor. This gauge allows assess-
ment of the performance of the primaries. A high pressure gauge is placed down-
stream of the booster compressor to assess the performance of the primaries and
booster when high pressure compressed air is required. Pressure gauges are also
placed upstream and downstream of the water injection pump and the solids
injector. These gauges allow assessment of the performance of these injection
systems. All these gauges must be high-quality gas gauges. Most drilling rig floors
are equipped with a mud pressure gauge. For air drilling operations this
mud gauge must be replaced with a high-quality gas gauge having the appropriate
pressure range.

3.2.7 Volumetric Flow Rate Meters
No driller would carry out a mud drilling operation without knowing the volu-
metric flow rate of mud being circulated to the well. The volumetric flow rate
from a mud pump can be assessed easily by either counting strokes per minute
of the mud pump (and knowing the capacity of the pump in gallons per stroke
and then calculating the output of the pump in gallons per minute) or by
providing the rig floor with an accurate volumetric flow rate gauge.
The volumetric flow rate of air (or other gases) to the well is vital knowledge
for a successful drilling operation and its knowledge must also be made available
to the rig personnel. The volumetric flow rate of air (or other gases) is referenced
to the atmospheric conditions of the air entering the primary compressor. At sea
level locations the volumetric flow rate is given as standard cubic feet per minute
(scfm) or standard liters per second and sometimes standard cubic meters per
second. At locations above sea level the volumetric flow rate is given as actual
cubic feet per minute (acfm) or as actual liters per second.
There are other very practical reasons why the compressed air flow to the dril-
ling operation must be metered. As will be seen in Chapter 5, there is a great deal
of difference in primary compressors and their respective effectiveness to pro-
duce the volume rate of air required by the operation. The screw compressor is
notorious for wearing and thus supplying less volumetric flow rate than adver-
tised. In addition, all compressors must be derated when the units are operated
at surface locations that are above sea level. This is one of the most important cal-
culations that must be carried out by the drilling engineer and must be verified by
on-site measurements.
There are two techniques for determining the air volumetric flow rate from
the primary compressors (or natural gas from a pipeline). A gas production orifice
plate with an associated recording system can be used in the flow line down-
stream of the compressors and scrubber, but upstream from the water injection
pump. Figure 3-3 shows a simple schematic of an orifice plate with a differential
pressure gauge to measure the difference between pressure upstream and down-
stream of the plate.