Valve Selection for Process Applications

Understanding the Critical Role of Valve Selection in Process Engineering

In any process application, valves serve as the essential control points that regulate flow, pressure, and direction of fluids throughout a system. Whether you're managing a water treatment facility in Halifax, operating a fish processing plant in Lunenburg, or running a pulp and paper mill in northern Nova Scotia, the valves you select can mean the difference between operational excellence and costly downtime.

Proper valve selection requires a thorough understanding of process conditions, fluid characteristics, and operational requirements. A valve that performs flawlessly in one application may fail catastrophically in another. This comprehensive guide will walk you through the critical considerations for selecting the right valve for your specific process application, with particular attention to the unique challenges faced by industries across Atlantic Canada.

Key Factors in Valve Selection: A Systematic Approach

Successful valve selection begins with a comprehensive analysis of your process requirements. Engineers must consider multiple variables simultaneously to ensure optimal performance, longevity, and safety. The following factors form the foundation of any valve selection process:

Process Fluid Characteristics

The nature of the fluid being controlled is perhaps the most critical consideration. Key characteristics include:

• Fluid type: Liquid, gas, steam, or multi-phase flow

• Viscosity: From water-thin fluids (1 cP) to heavy oils exceeding 10,000 cP

• Corrosivity: pH levels, chloride content, and chemical composition

• Abrasiveness: Suspended solids concentration and particle hardness

• Temperature: Operating range from cryogenic (-196°C) to high-temperature steam (540°C+)

• Toxicity and flammability: Safety classification requirements

For Maritime industries processing seawater, such as aquaculture operations common throughout Nova Scotia's coastal communities, the high chloride content (approximately 19,000 ppm) demands careful material selection. Standard carbon steel valves would experience rapid corrosion failure in such environments, necessitating duplex stainless steel, bronze, or specialized polymer materials.

Operating Conditions and Performance Requirements

Beyond fluid characteristics, engineers must thoroughly analyse operating conditions:

• Operating pressure: Normal operating pressure and maximum allowable working pressure (MAWP)

• Pressure drop: Available pressure differential across the valve

• Flow rate: Minimum, normal, and maximum flow requirements

• Control accuracy: Required rangeability and linearity

• Frequency of operation: Cycles per hour or day

• Fail-safe requirements: Fail-open, fail-closed, or fail-in-place

Common Valve Types and Their Optimal Applications

Understanding the strengths and limitations of each valve type enables engineers to match the right technology to specific applications. Here we examine the most common valve types encountered in process applications across Canadian industries.

Gate Valves

Gate valves remain the workhorse of on/off isolation applications. They provide minimal flow restriction when fully open, with typical Cv values 20-40% higher than globe valves of equivalent size. However, they are unsuitable for throttling service due to erosion concerns and poor control characteristics.

Ideal applications: Main isolation valves, pipeline applications, emergency shutdown systems

Specifications to consider: Rising stem versus non-rising stem, wedge type (solid, flexible, or split), seat material (metal-to-metal or resilient)

For applications in Nova Scotia's harsh winter climate, where ambient temperatures can drop below -30°C, specify valves with extended bonnets and low-temperature carbon steel or stainless steel construction meeting ASTM A352 LCC or LCB requirements.

Globe Valves

Globe valves excel in throttling applications where precise flow control is required. Their linear flow characteristic makes them predictable and easy to control, though they create significant pressure drop even when fully open.

Ideal applications: Flow regulation, pressure reduction, feedwater control

Key specifications: Trim type (contoured plug, V-port, or cage-guided), flow direction (flow-to-open or flow-to-close), stem packing material

In steam systems common to food processing facilities throughout the Maritimes, globe valves with stellite-faced trim can handle temperatures up to 565°C while providing excellent throttling control for process heating applications.

Ball Valves

Ball valves offer quick quarter-turn operation, tight shutoff, and relatively low cost. Modern designs with trunnion-mounted balls can handle pressures exceeding 700 bar (10,000 psi) in sizes up to 48 inches.

Ideal applications: On/off service, emergency isolation, hydrocarbon service

Considerations: Full-port versus reduced-port, floating versus trunnion-mounted, soft-seated versus metal-seated

For natural gas distribution applications in Atlantic Canada, fire-safe ball valves meeting API 607 and API 6FA standards provide critical protection against fire-induced seal failure.

Butterfly Valves

Butterfly valves provide excellent economy of space and weight, making them ideal for large-diameter applications where gate valves would be prohibitively expensive. A 24-inch butterfly valve typically weighs less than 200 kg, compared to over 1,000 kg for an equivalent gate valve.

Ideal applications: Water and wastewater treatment, HVAC systems, low-pressure isolation

Types: Concentric (resilient seated), double-offset (high-performance), triple-offset (metal-seated)

Municipal water treatment facilities across Nova Scotia frequently specify triple-offset butterfly valves for their ability to provide bubble-tight shutoff while handling the abrasive conditions created by treatment chemicals and sediment.

Check Valves

Check valves prevent reverse flow, protecting pumps, compressors, and process equipment from damage. Selection must account for cracking pressure, slam potential, and flow characteristics.

Types: Swing check, tilting disc, dual-plate (wafer), piston, and ball check designs

For pump discharge applications, dual-plate check valves with spring-assisted closure significantly reduce water hammer effects—a critical consideration for municipal pumping stations handling the variable demand patterns typical of Maritime communities.

Material Selection for Maritime and Canadian Conditions

The Atlantic Canadian environment presents unique challenges for valve materials. Coastal facilities must contend with salt-laden air, while all facilities face significant temperature variations throughout the year.

Body and Trim Materials

Standard material selections include:

• Carbon steel (ASTM A216 WCB): General service to 425°C, not suitable for corrosive environments

• Stainless steel (CF8M/316): Excellent corrosion resistance, suitable to 540°C

• Duplex stainless steel (CD3MN): Superior chloride resistance, ideal for seawater applications

• Bronze (C83600): Traditional choice for marine applications, limited to approximately 260°C

• Hastelloy C (CW-12MW): Severe corrosion service including acids and chlorides

Elastomer and Seal Selection

Seal materials must be compatible with process fluids and temperature extremes:

• EPDM: Excellent for water and steam service, -45°C to 150°C

• Viton (FKM): Hydrocarbon resistance, -20°C to 200°C

• PTFE: Universal chemical resistance, -200°C to 260°C

• Graphite: High-temperature service, fire-safe applications

For outdoor installations in Nova Scotia, elastomers must maintain flexibility at winter temperatures while resisting degradation from summer UV exposure. EPDM compounds formulated for cold weather service remain flexible to -50°C, providing reliable sealing throughout Atlantic Canada's variable climate.

Sizing and Performance Calculations

Proper valve sizing ensures optimal control performance while avoiding common problems such as cavitation, flashing, and noise.

Flow Coefficient (Cv) Calculations

The flow coefficient represents the flow rate in US gallons per minute of water at 60°F that produces a 1 psi pressure drop across the valve. For liquids:

Cv = Q × √(SG / ΔP)

Where Q is flow rate in GPM, SG is specific gravity, and ΔP is pressure drop in psi.

A properly sized control valve should operate between 50-80% open at normal flow conditions, leaving sufficient rangeability for process upsets without approaching either fully closed or fully open positions.

Cavitation and Flashing

When pressure drops below the fluid's vapour pressure, vapour bubbles form. If downstream pressure recovers above vapour pressure, these bubbles collapse violently (cavitation). If pressure remains below vapour pressure, the liquid partially vaporises (flashing).

Both phenomena cause severe damage to valve trim and should be addressed through:

• Multi-stage pressure reduction trim

• Cavitation-resistant materials (stellite, tungsten carbide)

• Proper valve positioning in the system

• Downstream pressure maintenance

Standards and Compliance Requirements

Canadian process facilities must comply with various standards governing valve selection, installation, and maintenance.

Applicable Standards

• CSA B51: Boiler, Pressure Vessel, and Pressure Piping Code (Canadian requirements)

• ASME B16.34: Valves—Flanged, Threaded, and Welding End

• API 600/602/603: Steel gate, globe, and check valves

• API 6D: Pipeline and piping valves

• MSS SP-25: Standard marking system for valves

• ISO 5211: Actuator mounting interface dimensions

Facilities in Nova Scotia operating under provincial Technical Safety regulations must ensure all pressure-retaining components, including valves, meet the requirements of the Boiler and Pressure Vessel Safety Regulations. Third-party certification from approved inspection agencies is typically required for ASME-stamped equipment.

Documentation Requirements

For critical applications, specify comprehensive documentation including:

• Material test reports (MTRs) with full traceability

• Hydrostatic test certificates

• Positive material identification (PMI) results

• Dimensional inspection reports

• Weld procedures and welder qualifications (where applicable)

Maintenance and Lifecycle Considerations

Valve selection should account for the complete lifecycle cost, not merely initial purchase price. A valve that costs 30% more initially but lasts three times longer represents significant savings over the equipment lifespan.

Preventive Maintenance Requirements

Different valve types demand varying maintenance approaches:

• Gate valves: Annual stem packing inspection, periodic seat lapping

• Globe valves: Trim inspection every 2-3 years, packing replacement as needed

• Ball valves: Seat seal replacement every 3-5 years depending on service

• Butterfly valves: Seat inspection annually, bearing lubrication for high-performance designs

Spare Parts Strategy

For facilities in Atlantic Canada, where shipping times from major manufacturing centres can extend lead times significantly, maintaining critical spare parts inventory becomes essential. Recommended spares include:

• Complete trim sets for control valves

• Packing sets and gaskets

• Seat rings and seal kits

• Actuator repair kits for automated valves

Partner with Sangster Engineering Ltd. for Your Valve Selection Needs

Selecting the optimal valve for your process application requires careful analysis of operating conditions, fluid properties, and performance requirements. The consequences of incorrect selection—from production losses to safety incidents—underscore the importance of getting it right the first time.

Sangster Engineering Ltd. brings decades of mechanical engineering expertise to process applications throughout Nova Scotia and Atlantic Canada. Our team understands the unique challenges faced by Maritime industries, from the corrosive coastal environment to the extreme temperature variations that characterize our region.

Whether you're designing a new process system, troubleshooting valve performance issues, or developing a valve maintenance programme, our professional engineers can provide the technical expertise you need. We offer comprehensive engineering services including process design, equipment specification, failure analysis, and ongoing technical support.

Contact Sangster Engineering Ltd. today to discuss your valve selection challenges and discover how our engineering expertise can optimise your process operations while ensuring safety, reliability, and regulatory compliance.

Partner with Sangster Engineering

At Sangster Engineering Ltd. in Amherst, Nova Scotia, we bring decades of engineering experience to every project. Serving clients across Atlantic Canada and beyond.

Contact us today to discuss your engineering needs.