Pressure and Flow
For most sampling applications, the following information is sufficient in determining the type of pump to select:
If the required vacuum is not known, you will need to calculate the pressure drop from the sample point to the pump inlet. ADI can perform this calculation. What we need to know is the required flow rate, line distance and diameter, Gas composition and temperature and initial pressure at the sample point.
If the sample gas is venting to atmosphere downstream, then only the vacuum needs to be calculated. If the gas is returning to a flare or back to process, then the pump will probably require pressure to get there. Again, ADI can calculate this – however we will need to know the pressure at the return point.
Reading the Curves
If the pump is located close to the sample point and the downstream pressure required is 5 PSIG @ 6 LPM, then all we need to do is find a pump that will provide ≥ 6 LPM at this pressure. See below.
Say an application requires a pump to pull through 200′ of 3/8″ sample line, flow to an analyzer, and then return to a process line 350′ away. In this case, the pump will be required to pull a vacuum on the inlet, provide flow to an analyzer, and provide positive pressure to the process line.
Combination Curve Example
In some cases, the process line may be under positive pressure when it reaches the pump inlet. If the gas sample must then be pushed back to a process line, a pump can be used to “boost” the pressure. When the pump inlet pressure exceeds 0 PSIG, the pump performance capabilities (in terms of discharge pressure and flow rate) can increase significantly. It is very important that under these circumstances specific calculations are done by ADI to avoid oversizing the pump; which can lead to premature diaphragm, bearing, and motor wear. The information we require in these applications (in addition to gas type and temperature) are pump inlet Pressure (PI) , Pump outlet Pressure (PO), and flow rate.