Pipeline gas is never perfectly clean. Rust and pipe scale, iron sulfide, dust, water, liquid hydrocarbons and compressor lube oil all travel with the gas. These particles and droplets build up on meter surfaces, wear the fine internals of regulators and break the single-phase flow condition that metering standards assume. A filtration and metering skid places the filters that capture these contaminants ahead of the metering section on one skid-mounted frame, protecting both measurement accuracy and the service life of downstream equipment.
How contaminants affect metering and regulating equipment
· Ultrasonic meters: Layers building up on transducer faces and the pipe wall change signal quality and the flow profile; contaminant buildup in the pipeline is one of the conditions known to affect ultrasonic metering accuracy.
· Orifice plates: Deposits on the upstream face or edge of the plate change the discharge coefficient and can bias the differential pressure measurement.
· Turbine and rotary meters: In meters with moving parts, solids can cause bearing wear and rotor damage.
· Pressure regulators: Regulators are generally designed for clean, dry gas; contamination reaching pilot circuits and seats undermines outlet pressure stability and tight shutoff.
· Analyzer systems: Sample lines for gas chromatographs and moisture analyzers are sensitive to liquid and particle carryover.
Liquid carryover is also a measurement issue in its own right, because differential pressure and ultrasonic metering standards are written for single-phase gas. Filtration is therefore designed not only to protect equipment but to keep the measurement within the conditions the standards assume.
Filter types and where they fit
Filter type | What it removes | Typical use |
Cartridge (dry gas) filter | Solid particles, using replaceable elements | Distribution and industrial inlets where gas is essentially dry |
Filter separator | First stage captures solids and coalesces droplets; second stage removes remaining liquid with a wire-mesh or vane mist extractor | Transmission connections, custody transfer points and sensitive consumers with a risk of liquid carryover |
Cyclone (centrifugal) separator | Uses rotational flow to separate heavy particles and liquid slugs; no filter element | Pre-separation stage on lines with heavy contaminant loads or sudden liquid arrivals |
These types are often combined rather than treated as alternatives. On a line with heavy liquid loading, for example, a cyclone pre-separator can be installed ahead of a filter separator. The right choice comes from considering the gas source, operating history and any sample analyses together, and filtration rating and separation efficiency should be compared on the basis of the manufacturer's stated test conditions.
Differential pressure: the filter's health indicator
As the element loads up, the pressure difference between filter inlet and outlet rises. It can be monitored with a local differential pressure gauge or with a transmitter connected to the control system. With a transmitter, the trend can be followed on SCADA and an alarm at a set threshold used to schedule an element change. An unexpected sudden drop may indicate a ruptured or bypassed element. Quick-opening closures are commonly used to speed up element changes, but the filter must always be isolated and depressurized before the closure is opened.
Drainage and liquid handling
Liquid collected in the sump of a filter separator or cyclone can be re-entrained into the gas if it is not drained regularly. Level is monitored with a level gauge, and draining can be manual or automatic through a dump valve operated by a level switch or level controller. Because drained liquid may contain hydrocarbons and contaminants, the collection tank, venting and waste handling are planned together with hazardous area and environmental requirements.
Duplex filter arrangement
Where supply must not be interrupted, two parallel filters are installed, one in service and one on standby. When the differential pressure threshold is reached, flow is switched to the standby unit and the filter taken out of service is safely depressurized for an element change. Changeover valve sequencing must prevent both filters from being closed at the same time, and blowdown and vent connections should be provided on the skid.
Flow sequence on the skid
1. Inlet isolation valve and, if needed, pressure measurement
2. Cyclone pre-separator where heavy liquid or particle loads are expected
3. Duplex filters or filter separators with differential pressure monitoring, drains and vents
4. Meter run with flow conditioner and straight pipe sections
5. Pressure and temperature measurement with a flow computer or volume corrector
6. Outlet isolation valve and data link to the control system
Filter pressure drop must be included in the skid's overall pressure loss budget. The difference between a clean element and one at the change-out threshold can become the deciding factor for downstream supply pressure, particularly in low-pressure distribution applications.
Design inputs
Parameter | Why it matters in design | Effect on selection |
Contaminant type and load | Solids and liquids need different separation mechanisms | Decides between cartridge filter, filter separator or cyclone |
Likelihood of liquid carryover | Sudden liquid arrival affects both filter and meter | Drives pre-separation, sump volume and automatic drainage |
Flow rate and operating pressure | Vessel diameter and element count depend on gas velocity | Sets filter size and number of parallel units |
Allowable pressure drop | Filter loss adds to metering and regulating losses | Sets element change threshold and vessel size |
Tolerance for interruption | Determines whether supply continues during element changes | Decides between single and duplex arrangement |
Downstream equipment | Meters and regulators differ in sensitivity to contaminants | Sets required filtration rating and monitoring level |
This skid differs from fuel gas conditioning, which prepares gas to meet the fuel specification of a specific turbine or engine. The aim here is to protect metering and regulating equipment from general contaminants, not to meet a machine's fuel requirements, and heating and pressure reduction are outside its scope.
Working with TLY Enerji on filtration and metering
TLY Enerji provides engineering support to assess the filtration arrangement and the metering technology together, based on the contaminant profile and the protection needs of downstream equipment. Depending on project scope, this can include supplying differential pressure and level instruments, valves and metering equipment, integrating differential pressure and drain signals into the control system, site installation, commissioning and maintenance supervision. The source of the filter vessels and the skid's responsibility boundary are defined in the project specification.
Information for an initial assessment
· Gas source and known contaminant types, with any sampling or maintenance observations
· Whether liquid slugs have reached the line in the past
· Flow range, operating and design pressure, temperature
· Type of meter and regulator downstream
· Acceptable combined pressure loss for filtration and metering
· Whether supply can be interrupted for element changes
· Site facilities for collecting and disposing of drained liquid
Connected solutions
· Natural gas metering stations: Meter run architecture and redundancy after filtration.
· Fuel gas conditioning and metering skids: Gas preparation to a turbine or engine fuel specification.
· Gas pressure regulating skids: Design of regulator runs protected by filtration.
· Ultrasonic gas metering skids: Installation conditions for deposit-sensitive ultrasonic meters.
Filtration and metering skids: FAQ
Cartridge filter or filter separator?
If the gas is essentially dry and the problem is limited to solid particles, a cartridge filter is often sufficient. If water, liquid hydrocarbons or oil droplets are expected, a two-stage filter separator that captures solids and coalesces and removes droplets is more appropriate. The decision is made by weighing historical data on the gas source against the sensitivity of the equipment downstream.
When should a filter element be replaced?
Replacement is usually driven by the differential pressure across the filter rather than by the calendar. A threshold is defined that reflects the manufacturer's recommendation and fits the skid's pressure loss budget, and a change is scheduled when it is reached. An unexpected fall in differential pressure calls for a separate check, since it may point to element damage or bypassing.
Does filtration directly affect metering accuracy?
Indirectly, but significantly. The filter does not measure anything, yet by limiting deposits on meter surfaces, fouling of orifice plates and liquid entering the meter run, it keeps the meter operating close to standard conditions and to its behavior at calibration. In a poorly filtered meter run, the resulting error is often only discovered at the next inspection or calibration.
Is a duplex filter needed on every project?
No. Where short interruptions are acceptable and element changes can be done during planned shutdowns, a single filter may be enough. At power plants, industrial processes or network connections where supply cannot stop, a duplex arrangement allows elements to be changed without interrupting operation. The choice weighs the cost of an outage against the extra equipment and space.