A pressure gauge on only one side of a filter housing cannot directly show the pressure loss across the filter. For condition monitoring, buyers need a defined upstream and downstream sensing arrangement, either two matched gauges, a differential-pressure gauge/transmitter, or approved sensing points connected to plant instrumentation. The RFQ must specify where those readings are taken and what is included in the supplier’s scope.
This checklist applies to new and replacement single bag filter housings and multi bag housings. It complements the operating-versus-design-pressure data sheet; it does not establish a universal change-out differential pressure.
What differential pressure means
Differential pressure, ΔP, is upstream pressure minus downstream pressure at the same operating condition. The reading includes losses through the installed media and any flow restrictions between the sensing locations. It changes with flow, viscosity, temperature, media loading and valve position. Therefore a useful baseline always records those conditions with a clean element.
Parker literature illustrates two common approaches: sensing inlet and outlet pressure with two gauges, or using a dedicated differential-pressure gauge. Other Parker product literature also describes separate pressure gauge or pressure switch ports for visual or electrical monitoring. These examples support the instrumentation concept, but the allowable ranges and connection details must come from the actual housing and instrument data sheets.
Specify sensing point locations
- Locate the upstream point where it represents housing inlet pressure, not pump discharge pressure through a long pipe run.
- Locate the downstream point where it represents housing outlet pressure before another control valve or major restriction.
- Show both connections on the P&ID and the housing general arrangement drawing.
- State whether impulse tubing, root valves, manifolds, gauges, transmitters and electrical switches are in supplier or purchaser scope.
- Define accessibility for calibration, venting and safe maintenance.
If sensing points are too remote, the measured value can include unrelated pipe, valve or fitting losses. If one point is at a different elevation, a liquid-filled impulse line can introduce static-head effects. The plant’s instrumentation engineer should review these details, particularly for dense liquids, slurry service or long impulse lines.
RFQ data for taps and instruments
| Field | Buyer should state |
|---|---|
| Process | Liquid, solids, viscosity, temperature and compatibility concerns |
| Pressure basis | Normal/minimum/maximum operating pressure and design pressure |
| Flow | Minimum, normal and maximum flow |
| Tap connection | Thread or flange standard, nominal size, orientation and plug requirement |
| Instrument | Two gauges, DP gauge, switch or transmitter; local or remote indication |
| Range | Expected clean and loaded ΔP; selected by the instrumentation engineer |
| Materials | Wetted parts, seals and impulse tubing compatible with the process |
| Protection | Isolation, overpressure, pulsation damping, flushing or heat tracing if required |
| Documentation | P&ID tag, calibration certificate, datasheet and wiring details |
Do not copy a change-out value from another housing
Published instructions for one branded system may state a specific element replacement differential pressure. That value belongs to that equipment and application. For a bag housing, the allowable clean and terminal ΔP must be confirmed against the bag, restrainer basket, seals, vessel design and process requirements. Some processes change bags earlier to protect product quality or maintain flow; others are limited by pump or downstream equipment.
Request three distinct values in the supplier response: clean ΔP at the stated liquid and flow, the recommended operating/change-out criterion for the quoted element, and the maximum allowable differential pressure for the weakest relevant component. These are not interchangeable.
Commissioning and alarm logic
- Verify tap labels and the actual flow direction.
- Leak-test connections under the approved procedure.
- Equalize or zero the DP device according to its manufacturer’s instructions.
- Vent air from liquid impulse lines where the design requires it.
- Record a clean baseline at stable temperature and several defined flow points.
- Check that an alarm or switch cannot be defeated by a closed isolation valve.
- Document the operator response to high ΔP; do not rely on an alarm without a safe change-out procedure.
For the vessel itself, also include vent and drain requirements from the vent and drain RFQ checklist, and keep design conditions separate using the filter housing pressure data sheet.
Request a quote with a complete instrument boundary
Request a Quote with the housing size, liquid, temperature, flow, pressure data, connection standard, desired indication and plant instrument tag. Ask for a marked drawing that identifies inlet tap, outlet tap, instrument scope and orientation before approval.
FAQ
Can one inlet pressure gauge show filter differential pressure?
No. Differential pressure requires pressure information from both sides of the relevant filter restriction, or a dedicated differential-pressure device connected across it.
Should the DP taps be at the pump discharge and process outlet?
Usually not if those points include unrelated piping or valves. The locations should represent pressure immediately upstream and downstream of the housing as defined on the P&ID.
Is there one standard filter bag change-out DP?
No. Confirm the criterion for the quoted bag, housing and process. Flow, viscosity, loading, basket support and product-quality requirements all matter.
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