Fuel Gas Filter Design

1. Fuel Gas Filtration Overview

Fuel gas filtration is critical for protecting gas-consuming equipment from damage caused by particulate matter, liquid droplets, and aerosol contaminants. Gas engines, gas turbines, burners, and catalytic heaters all require clean, dry fuel gas to operate reliably and efficiently. Even small amounts of liquid or solid contamination can cause significant damage to combustion components, control valves, and fuel nozzles.

Contaminants in Fuel Gas

Natural gas fuel systems commonly encounter these contaminants:

Contaminant	Source	Effect on Equipment
Pipeline dust and scale	Corrosion products, construction debris	Erosion of fuel valves, nozzle plugging
Compressor oil	Reciprocating compressor carryover	Fouling of combustion surfaces, carbon deposits
Condensate (hydrocarbon liquid)	Retrograde condensation, JT cooling	Flame instability, hot-section damage
Water (liquid and vapor)	Pipeline condensation, wellstream	Corrosion, flame-out, ice formation
Glycol carryover	Dehydration unit upset	Fouling, carbon deposits, catalyst poisoning
Amine carryover	Sweetening unit upset	Corrosion, combustion chamber deposits
Sand and fines	Production wells, pipeline erosion	Severe erosion of all wetted parts

Filtration System Components

A complete fuel gas filtration system typically includes:

• Inlet separator/scrubber: Bulk liquid removal upstream of the filter vessel
• Filter/coalescer vessel: Pressure vessel containing replaceable filter elements
• Differential pressure instrumentation: Monitors filter element condition
• Drain system: Automatic or manual liquid drain from the filter vessel sump
• Pressure regulator: Downstream pressure reduction to engine/turbine fuel pressure
• Fuel gas heater (optional): Prevents hydrocarbon condensation after pressure reduction

2. Filter Element Types

Fuel gas filters use replaceable cartridge elements that perform either particulate removal, liquid coalescence, or both. The element type must match the contaminants present and the downstream equipment fuel gas specifications.

Particulate Filter Elements

Particulate filters remove solid particles by depth filtration or surface filtration. Gas flows from outside to inside through the filter media, trapping particles within or on the surface of the element.

• Pleated cellulose: Low cost, 3–10 μm rating, moderate dirt-holding capacity. Standard for gas engine fuel gas.
• Pleated synthetic (polyester/polypropylene): Better moisture resistance than cellulose, 1–10 μm rating. Good for wet gas service.
• Pleated glass fiber: High efficiency (0.3–3 μm), good dirt-holding capacity. Required for gas turbine fuel gas.
• Sintered metal: Cleanable and reusable, 2–25 μm rating. Higher initial cost but eliminates element replacement.
• Wrapped depth media: Multiple layers of graduated density. Good for high-particulate loading applications.

Coalescing Filter Elements

Coalescing elements remove liquid aerosols by capturing tiny droplets on fine fibers, merging them into larger droplets that drain by gravity. Flow direction is typically inside-out to prevent re-entrainment.

• Glass microfiber: Standard coalescing media, removes liquid aerosols down to 0.3 μm. Rated for 99.5%+ liquid removal efficiency.
• Two-stage coalescer: First stage coalesces fine aerosols; second stage (separator element) prevents re-entrainment of coalesced droplets.
• Combination elements: Single element with both particulate and coalescing capability. Common for compact fuel gas skids.

Element Selection Guide

Application	Element Type	Rating (μm)	Notes
Gas engine (low speed)	Pleated cellulose/synthetic	5–10	Cost-effective, adequate for most engines
Gas engine (high speed)	Pleated synthetic or glass fiber	3–5	Tighter filtration for high-speed valve protection
Gas turbine	Coalescing + glass fiber	0.3–1	OEM requirement; must meet ISO 8573 or equivalent
Catalytic heater	Pleated cellulose	5–10	Protect catalyst bed from particulates
Burner / flare	Pleated cellulose	10–25	Basic particulate removal adequate
Wet gas service	Coalescing (two-stage)	0.3–3	Liquid removal is primary concern

Element Performance Characteristics

Parameter	Particulate Element	Coalescing Element
Clean ΔP	0.5–2 psid	1–3 psid
Change-out ΔP	8–15 psid	10–15 psid
Flow direction	Outside-in	Inside-out
Element life	6–18 months	6–12 months
Liquid tolerance	None (will saturate)	Designed for continuous liquid
Solid particle efficiency	95–99.97%	95–99%
Liquid aerosol efficiency	Minimal	99.5–99.98%

3. Fuel Gas Specifications

Gas engine and turbine manufacturers specify fuel gas quality requirements to protect their equipment and maintain warranty coverage. These specifications define allowable levels of particulates, liquids, and chemical contaminants.

Gas Engine Fuel Gas Requirements

Parameter	Typical Requirement	Notes
Particulate size	< 5 μm	Some high-speed engines require < 3 μm
Particulate loading	< 15 ppmw	By weight in the gas stream
Free liquid	None	No visible liquid droplets
Liquid aerosol	< 0.003 ppmw	Coalescing filter required
H2S	< 100–800 ppm	Varies by manufacturer; affects spark plug life
Total sulfur	< 1,000 ppm	Affects catalyst and oil life
Fuel pressure	3–75 psig	Per engine model; typically 30–50 psig
Superheat	> 20°F above dewpoint	Prevents condensation in fuel system

Gas Turbine Fuel Gas Requirements

Parameter	Typical Requirement	Notes
Particulate size	< 0.3–1 μm	Much tighter than gas engines
Free liquid	None	Zero tolerance for liquid in fuel nozzles
Liquid aerosol	< 0.003 ppmw	Two-stage coalescing required
Na + K	< 0.5 ppmw	Causes hot corrosion of turbine blades
Pb	< 1 ppmw	Lead compounds foul turbine internals
V	< 0.5 ppmw	Vanadium causes severe hot corrosion
H2S	< 20–200 ppm	Tighter than gas engines; varies by OEM
Superheat	> 50°F above dewpoint	Greater margin than gas engines
Fuel pressure	300–600 psig	Depends on turbine model and combustion system

Fuel Gas Heating Requirements

When fuel gas pressure is reduced through a regulator, the Joule-Thomson (JT) cooling effect can drop the gas temperature below the hydrocarbon dewpoint, causing condensation. A fuel gas heater is required when:

4. Sizing Methodology

Fuel gas filter vessels are sized based on the gas flow rate, operating pressure, allowable pressure drop, and the number of filter elements required to achieve the design service life.

Step 1: Determine Gas Flow Rate

Step 2: Calculate Actual Volume Flow

Step 3: Select Element Size and Count

Filter elements are rated for a maximum face velocity that determines how much gas each element can handle. The number of elements required is:

Step 4: Select Vessel Size

Vessel Size (inches)	Element Count	Typical Flow Capacity (ACFM)
8–12	1–3	50–400
16–20	4–9	400–1,200
24–30	10–19	1,200–3,000
36–42	20–37	3,000–6,000
48–60	37–61+	6,000–12,000

Pressure Drop Calculation

5. Monitoring & Controls

Differential pressure monitoring is the primary method for determining filter element condition. A well-designed monitoring system provides early warning of element loading and prevents operation with failed or bypassed elements.

Differential Pressure Monitoring

ΔP Level (psid)	Status	Action
0.5–3	Clean elements	Normal operation
3–5	Partially loaded	Monitor trending; plan element order
5–8	Approaching change-out	Order replacement elements; schedule change-out
8–12	Change-out required	Replace elements at next opportunity
> 12–15	Overdue / element failure	Immediate replacement; investigate cause of rapid loading

Recommended Instrumentation

• Differential pressure gauge: Local visual indication of element ΔP. Install at eye level on the filter vessel.
• Differential pressure transmitter: 4–20 mA signal to DCS/PLC for trending, alarming, and automated actions.
• High ΔP alarm: Set at 80% of maximum allowable ΔP (typically 8–10 psid).
• High-high ΔP shutdown: Set at maximum allowable ΔP (12–15 psid). Protects against element collapse and bypass.
• Liquid level sensor: Monitors liquid accumulation in filter vessel sump. Triggers automatic drain or alarm.
• Temperature transmitter: Monitors fuel gas temperature for dewpoint margin verification.

Automatic Drain Systems

Liquid collected by coalescing elements accumulates in the vessel sump and must be removed. Options include:

• Manual drain valve: Simplest option; operator opens valve periodically. Risk of overfilling sump.
• Float-operated drain: Mechanical float valve opens automatically when liquid reaches a set level. Reliable for small volumes.
• Electronic level-controlled drain: Level transmitter operates a control valve or solenoid. Best for high-liquid-loading applications.
• Timer-based drain: Solenoid valve opens on a timed cycle. Simple but may drain gas or miss high-liquid events.

6. Worked Example

Size a fuel gas coalescing filter for a 2,000 HP natural gas compressor station with two gas engine drivers.

Step 1: Calculate Fuel Gas Flow Rate

Step 2: Convert to Actual Conditions

Step 3: Select Elements

Step 4: Select Vessel

Step 5: Verify Pressure Drop

Summary

Parameter	Value
Fuel gas flow rate	278 SCFM (36.9 ACFM at filter)
Element type	Coalescing, 3 μm glass microfiber
Elements per vessel	2
Vessel size	12" dia × 48" shell
Design pressure	150 psig
Clean ΔP	~0.5 psid
Change-out ΔP	10 psid
Configuration	Duplex with transfer valve

7. Operations & Maintenance

Element Replacement Procedure

1. Verify system can be isolated (duplex: switch to standby vessel; simplex: shut down equipment)
2. Depressurize the filter vessel through the vent valve
3. Open the vessel closure (quick-opening closure or bolted cover)
4. Remove spent elements and inspect for damage, bypass evidence, or unusual deposits
5. Clean the vessel interior and inspect drain screens
6. Install new elements; verify proper seating and O-ring compression
7. Close vessel, pressurize slowly, and check for leaks
8. Verify clean ΔP is within expected range (0.5–3 psid)
9. Return vessel to service and record the change-out in the maintenance log

Common Problems and Solutions

Problem	Likely Cause	Solution
Rapid element loading	Upstream process upset, pipeline pig	Investigate and correct source; add pre-filter
High clean ΔP on new elements	Wrong element type, oversized flow	Verify element rating; add elements or larger vessel
Liquid passing through	Element saturated, wrong element type	Switch to coalescing element; check drain system
Zero ΔP	Element failure, bypass, no flow	Inspect elements; check for bypass around seals
Sump overfilling	Drain valve failure, high liquid loading	Repair drain; add upstream scrubber
Engine/turbine fouling despite filter	Contaminant passing through filter	Tighten filtration rating; add coalescing stage

Spare Parts Inventory

Item	Recommended Stock
Filter/coalescing elements	2 full sets minimum
O-rings (element and closure)	2 sets per vessel
Closure gaskets	4 per vessel
Drain valve repair kit	1 per vessel
ΔP gauge	1 spare

Maintenance Schedule

Task	Frequency
Check ΔP gauge/transmitter	Daily (or continuous via DCS)
Drain sump liquid	Daily (manual) or automatic
Inspect drain valve operation	Weekly
Record ΔP trending data	Weekly
Element replacement	When ΔP reaches 8–12 psid
Vessel internal inspection	Annually or at element change
Pressure relief valve inspection	Annually