Bacharach HI FLOW Sampler, Instruction

Page: 5 / 72

Hi Flow Sampler Introduction
Instruction 0055-9017
1
1 Introduction
1.1 General Description
The Hi Flow Sampler is portable, intrinsically safe, battery-powered
instrument designed to determine the rate of gas leakage around various
pipefittings, valve packings, and compressor seals found in natural gas
transmission, storage, and compressor facilities.
A component’s leak rate is measured by sampling at a high flow rate so
as to capture all the gas leaking from the component along with a certain
amount of surrounding air. By accurately measuring the flow rate of the
sampling stream and the natural gas concentration within that stream,
the gas leak rate can be calculated using Equation 1. The instrument
automatically compensates for the different specific gravity values of air
and natural gas, thus assuring accurate flow rate calculations.
Leak = Flow x (Gas
sample
– Gas
background
) x 10
–2
Eq. 1
where:
Leak = rate of gas leakage from source (cfm)
Flow = sample flow rate (cfm)
Gas
sample
= concentration of gas from leak source (%)
Gas
background
= background gas concentration (%)
To ensure that the instrument is capturing all the gas that is escaping
from the component, two measurements are performed at two different
flow rates. The first measurement is taken at the highest possible flow
rate, followed by a second measurement at a flow rate that is
approximately 70–80% of the first. If the two calculated leak rates are
within 10% of each other, then it can be assumed that all gas has been
captured during the test.
For those mathematicians who are wondering how both calculations can
be the same with different flow rates, note that the measured CH
4
level at
the lower flow rate will be higher due to less air being drawn in with the
sample, thus balancing out the lower flow rate.
The instrument is packaged inside a backpack, thus leaving the
operator’s hands free for climbing ladders or descending into manholes.
The instrument is controlled by a handheld unit consisting of an LCD
and a 4-key control pad, which is attached to the main unit via a 6 foot
coiled cord.
The gas sample is drawn into the unit through a flexible 1.5 inch I.D.
hose. Various attachments connected to the end of the sampling hose
provide the means of capturing all the gas that is leaking from the
component under test.
The main unit consists of an intrinsically safe, high-flow blower that
pulls air from around the component being tested through a flexible hose
and into a gas manifold located inside the unit. The sample is first
passed through a venturi restrictor where the measured pressure
differential is used to calculate the sample’s actual flow rate. Next, a
portion of the sample is drawn from the manifold and directed to a
combustibles sensor that measures the sample’s CH
4
concentration in