Varec Biogas (Ovivo) vs Shand & Jurs for Digestion

Introduction

The safety and structural integrity of anaerobic digesters rely entirely on the precise management of internal pressures. For decades, the specification of digester gas safety equipment has largely been a choice between two established North American suppliers. Engineers evaluating Varec Biogas (Ovivo) vs Shand & Jurs for Digestion applications are navigating a landscape defined by legacy installations, subtle differences in sealing technology, and critical reliability requirements. Both are genuine specialists in this equipment class — which makes this one of the few head-to-head comparisons in digestion where the two companies actually compete for the same scope. The nuances in their pressure/vacuum relief valve (PVRV) designs, flame arrester technologies, and waste gas burner configurations lead to real differences in operating expenditure and site safety profiles.

Digester covers—whether floating, fixed, or membrane—operate within extremely narrow pressure bands, often measured in inches of water column. A failure in the relief system can lead to structural failure of the cover or implosion under vacuum conditions. Furthermore, the biogas environment is inherently hostile, characterized by high moisture, hydrogen sulfide (H₂S), and siloxanes, which challenge the materials and moving parts of safety valves.

This article provides a neutral engineering analysis of the equipment offered by these two OEMs. By moving beyond brochure claims and focusing on mechanical design, leakage rates, maintenance accessibility, and API 2000 compliance, this guide aims to assist municipal consulting engineers and plant directors in making data-driven specification decisions for biogas handling systems. It sits within the wider field of digester manufacturers, where the gas train is typically bid as a separate package from the tank and mixing equipment.

How to Select / Specify

Selecting the correct gas safety equipment requires a granular understanding of process variables. When comparing Varec Biogas (Ovivo) vs Shand & Jurs for Digestion, engineers must look past the brand name to the specific mechanical attributes that align with the facility’s risk profile and maintenance capabilities.

Duty Conditions & Operating Envelope

The primary driver for selection is the operating pressure relative to the maximum allowable working pressure (MAWP) of the digester tank or cover. Digesters typically operate at 6 to 12 inches w.c., with relief set points only marginally higher.

• Set Pressure vs. Operating Pressure: A critical specification parameter is the margin between operating pressure and the relief valve set point. If the margin is too narrow, valves may simmer, leading to fugitive emissions and accelerated seat wear.
• Flow Capacity (SCFH): Sizing must account for both normal venting (gas production plus pump-in displacement) and emergency venting (fire exposure). Both suppliers publish capacity curves, and these must be corrected for the density of biogas rather than air. For a typical 60-65% methane biogas the molecular weight works out near 26-27, giving a specific gravity relative to air in the neighbourhood of 0.9. Calculate it from the actual gas analysis rather than adopting a rule-of-thumb figure, because the correction moves with methane fraction.
• Vacuum Requirements: Vacuum relief is often the more critical scenario, preventing cover or tank implosion during rapid sludge withdrawal. The valve’s flow characteristics at low vacuum settings must be verified against pump-out rates.

Materials & Compatibility

Biogas is wet and corrosive. The presence of H₂S necessitates strict material controls. Standard aluminum construction is often sufficient for low-sulfide gas, but higher H₂S requires upgrades.

• Body Materials: Low-copper aluminum is the industry standard for housings. However, coastal plants or high-H₂S environments may require 316 stainless steel bodies, which significantly impacts CAPEX.
• Trim and Seat Materials: This is a key differentiator. Pallets and stems should be 316SS. The soft goods (diaphragms, seat inserts) must be compatible with the chemical composition of the gas. PTFE and FEP are common for their chemical resistance and non-stick properties, which matter for preventing freezing or sticking.
• Weights: Lead weights are traditional but can corrode or deform. Zinc-plated steel or encapsulated weights are preferred to maintain precise set pressures over decades.

Hydraulics & Process Performance

The tightness of the valve as it approaches the set point defines its performance.

• Leakage Rates: Engineers should specify maximum allowable leakage rates at 75% or 90% of set pressure. API 2000 provides guidelines, and high-performance valves from both suppliers can exceed them.
• Blowdown: This refers to the pressure difference between the set pressure and the pressure at which the valve reseats. A short blowdown preserves valuable biogas; a long blowdown wastes gas and lowers system pressure unnecessarily.
• Overpressure: To achieve full rated flow, PVRVs require overpressure (accumulation). Engineers must ensure the tank MAWP is not exceeded at the required flow rate, which usually occurs at 10% to 100% overpressure depending on the valve technology.

Installation Environment & Constructability

• Pipe-Away vs. Vent-to-Atmosphere: Valves venting to atmosphere are simpler but may cause odor issues. Pipe-away configurations require careful backpressure calculations, as downstream piping friction increases the effective opening pressure.
• Winterization: In northern climates, freezing condensate is a primary failure mode. Insulation jackets and steam or electric tracing are mandatory. Both manufacturers offer all-weather versions, but the effectiveness of the thermal design varies.
• Weight and Structural Load: Large diameter valves (10″+) are heavy. Structural engineers must account for point loads on digester covers or piping supports.

Reliability, Redundancy & Failure Modes

The most common failure mode for digester safety valves is sticking closed due to struvite crystallization, siloxane buildup, or freezing.

• Sticking: Design features that minimize surface area contact between the pallet and the seat reduce the risk of sticking.
• Redundancy: It is standard engineering practice to install PVRVs in pairs (one duty, one standby) or to provide a separate emergency relief manway. This allows one unit to be isolated for maintenance without taking the digester offline.

Controls & Automation Interfaces

• Position Indication: Limit switches or proximity sensors can be specified to alert SCADA when a relief valve lifts. This provides early warning of system pressure imbalances or control valve failures downstream.
• Manometers: Local pressure indication integrated into the valve body or adjacent piping is essential for operator rounds.

Maintainability, Safety & Access

• Component Accessibility: Can the pallet assembly be removed without taking the entire valve body off the mounting flange?
• Cleaning: Designs that allow for easy scraping or power-washing of the seat area are preferred.
• Confined Space: Valves located in the center of floating covers present significant access challenges. Edge-mounted valves or davit-arm access points are preferable.

Lifecycle Cost Drivers

• Initial Cost: Generally comparable between the two for standard specifications.
• Maintenance Labor: The primary OPEX driver. Valves that require monthly cleaning versus quarterly cleaning significantly change the lifecycle cost.
• Gas Loss: Leaky valves result in lost energy potential. Premium sealing technologies pay for themselves through increased methane capture.

The Relief Valve Is Not the Whole Protection Scheme

A specification that treats the PVRV as the digester’s sole overpressure protection has a gap in it, and the gap is usually on the emergency side.

Normal venting and emergency venting are different problems with different magnitudes. Normal venting handles gas production plus liquid displacement during pump-in, and for a municipal digester that is a modest flow that a correctly sized PVRV passes comfortably. Emergency venting — typically driven by external fire exposure under API 2000 — can require an order of magnitude more capacity. A PVRV sized for normal venting will not pass the emergency case, and adding overpressure to force it through will exceed the structural rating of a concrete cover long before the valve reaches full flow.

That is what the emergency relief manway is for. It is a large, low-set-pressure device, usually a weighted hatch, that opens and stays open. It is not redundancy for the PVRV; it is a separate device answering a separate question, and it must be in the design and in the hydraulic calculation. A plant with two PVRVs and no emergency vent has redundancy against one failure mode and no protection against the other.

Two downstream items complete the scheme and are routinely undersized. The flame arrester’s pressure drop at full flow subtracts directly from the available accumulation, so a valve set at 10″ w.c. on a cover rated for 12″ w.c. has 2″ of margin, and a flame arrester taking 3″ at rated flow has consumed all of it. Calculate the whole path, not the valve alone. And in pipe-away arrangements, condensate collecting in a low point adds static backpressure that raises the effective set pressure of the valve with no indication at the panel. Drip traps at every low point, drained on a schedule, are part of the safety system rather than a housekeeping detail.

Comparison Tables

The following tables provide a side-by-side analysis. Table 1 compares the manufacturers’ general approach to the digestion market. Table 2 provides a technical feature matrix for their pressure/vacuum relief valves, the most critical safety component. Model designations change; verify current series with the manufacturer before specifying.

Engineer & Operator Field Notes

Drawing from commissioning reports and long-term operational logs, the following insights highlight practical considerations often missed in the initial specification of Varec Biogas (Ovivo) vs Shand & Jurs for Digestion systems.

Commissioning & Acceptance Testing

The transition from construction to operation is where most errors in gas safety systems are discovered.

• Weight Verification: A common failure during site acceptance testing is incorrect set pressure. Contractors often ship valves with weights packaged separately. Verify the actual weight stack against the stamped nameplate and the system design pressure. Do not assume the factory setting is correct after shipping and handling.
• Shipping Blocks: Both manufacturers use shipping blocks or inserts to protect the seating surfaces during transport. Pro Tip: Create a specific punch-list item to verify removal of all internal packing material before gas introduction.
• Leak Testing: Do not rely solely on the manufacturer’s shop test. Perform a low-pressure air test on the digester with the valves installed. Use leak detection fluid on the flange faces and the valve outlet to check for premature lifting.

Common Specification Mistakes

• Specifying “Air” Capacity: Engineers sometimes specify flow rates in SCFM of air without correcting for biogas density. This is conservative in direction but can lead to oversized valves, which are more expensive and prone to chatter.
• Ignoring Flame Arrester Pressure Drop: When a PVRV is mounted in line with a flame arrester, the pressure drop across the arrester must be subtracted from the available overpressure. A valve set at 10″ w.c. protecting a tank rated for 12″ w.c. leaves only 2″ w.c. for accumulation. If the flame arrester creates 3″ w.c. drop at maximum flow, the tank is unprotected.
• Omitting the Emergency Vent: A PVRV sized for normal venting cannot pass the API 2000 fire case. The emergency relief manway is a separate required device, not optional redundancy.
• Material Mismatch: Specifying aluminum internals for a system with high H₂S will lead to rapid corrosion and seizure. 316SS trim should be the minimum standard for internal wetted parts in wastewater digestion.

O&M Burden & Strategy

Operators must understand that these are not install-and-forget devices.

• Struvite & Condensate: In colder months, warm saturated gas hits the cold valve body, creating condensate and potentially struvite crystals. This effectively glues the pallet to the seat.
• Maintenance Interval:
  • Monthly: Visual inspection and check for icing in winter.
  • Quarterly: Exercise the valve by manual lift to ensure freedom of movement.
  • Annually: Full tear-down, cleaning of seats with non-abrasive solvent, and inspection of diaphragms and O-rings.
• Spare Parts: Keep a full set of soft goods and at least one spare pallet assembly on the shelf. Lead times for OEM parts are long.

Troubleshooting Guide

• Valve chatter (rapid opening and closing): Usually indicates the valve is oversized for the flow rate, or the inlet piping is too restrictive.
• Continuous leakage: Check for debris on the seat. If the seat is clean, check whether the operating pressure is too close to the set pressure.
• Sticky operation: Likely caused by siloxane or struvite buildup. Requires solvent cleaning. Do not use metal scrapers on PTFE seats.

Design Details / Calculations

Proper sizing safeguards the physical assets. The methodology generally follows API 2000 / ISO 28300, tailored for low-pressure storage.

Sizing Logic & Methodology

The required venting capacity is the sum of two components:

1. Normal Venting:
   • Liquid movement: maximum pump-in rate converted to gas displacement. One gallon is 0.1337 ft³, so 1 GPM displaces approximately 8.02 SCFH.
   • Thermal effects: expansion due to temperature rise, less critical for insulated digesters but relevant for gas holders.
   • Gas production: peak biological gas generation rate.
2. Emergency Venting:
   • Fire exposure: heat input from an external fire causes rapid gas expansion. API 2000 provides formulas based on wetted surface area.
     Rule of Thumb: For many municipal digesters, the emergency venting requirement is far larger than the normal venting requirement, which is why it needs its own device.

Specification Checklist

To ensure a robust procurement, the specification document must include:

• Fluid Service: Anaerobic digester gas, with composition given (%CH₄, %CO₂, ppm H₂S, moisture content).
• Set Pressure & Vacuum: Defined in inches w.c. or mbar.
• Maximum Allowable Working Pressure: Of the tank and cover, stated separately if different.
• Leakage Class: A numeric allowance at a stated percentage of set pressure.
• Emergency Relief Device: Separately specified and sized for the API 2000 fire case.
• Certifications: Approvals for electrical components such as switches and trace heating in the applicable classified area.
• Testing: Requirement for certified test reports for leakage and set pressure.

Standards & Compliance

• API 2000 / ISO 28300: Venting of atmospheric and low-pressure storage tanks. The governing reference for sizing; use the current edition.
• NFPA 820: Standard for Fire Protection in Wastewater Treatment and Collection Facilities. Defines hazardous area classifications which dictate electrical requirements for valve accessories.
• ISO 16852: Relevant where flame arresters are integrated into the relief assembly.

FAQ Section

What is the main difference between Varec and Shand & Jurs PVRVs?

The primary technical difference lies in the sealing technology. Shand & Jurs uses a diaphragm-assisted seal energized by tank pressure, creating a tighter seal closer to the set point. Varec typically uses a weight-loaded pallet with an O-ring or soft insert. The Varec design is simpler and robust; the Shand & Jurs design offers better leakage performance but introduces a diaphragm as a wear item.

How do you select the correct size for a digester relief valve?

Selection is based on API 2000 calculations. Calculate the required venting capacity for liquid pump-in and pump-out, thermal expansion and contraction, biological gas production, and emergency fire relief. Once the total is determined, select a valve size that can pass the normal-venting flow at a pressure typically no higher than 10-20% above the set pressure, and provide a separate emergency relief device for the fire case.

Why do digester relief valves fail?

The most common causes are sticking due to struvite or frozen condensate, and corrosion. Biogas contains moisture and hydrogen sulfide; if the valve body is not heated or insulated in winter, ice forms. If the materials are not appropriate for sour service, H₂S attacks them. Lack of seat cleaning leads to leakage or to the valve failing to open at set pressure.

What is the typical lifespan of these valves?

With proper maintenance, the cast body housings of both manufacturers’ valves can last 20+ years. Internal soft goods (diaphragms, gaskets, O-rings) typically have a 3-5 year service life, and pallet assemblies may need refurbishment or replacement every 7-10 years depending on H₂S exposure.

Should I use a flame arrester with my PVRV?

Yes, if the PVRV vents to atmosphere where an external ignition source could ignite the gas plume. The flame arrester prevents the flame front from travelling back into the digester. It adds pressure drop, which must be included in the accumulation calculation, and requires frequent cleaning to prevent clogging that would block the relief path.

How do the costs compare?

Base pricing is broadly comparable for standard municipal specifications in the same size and material. Pricing moves with materials (aluminum versus stainless) and accessories (insulation jackets, position switches) more than with brand. Evaluate total cost of ownership based on maintenance frequency and the cost of replacement parts — diaphragms versus O-rings — rather than the quoted unit price.

How does this comparison relate to the mixing and pumping decisions?

They are separate scopes bid separately. The mixing technology choice is worked through in Varec Biogas (Ovivo) vs Smith & Loveless, the hydraulic-versus-gas-train interface in Smith & Loveless vs Shand & Jurs, and the wider supplier landscape in the review of digestion system OEMs.

Conclusion

The choice between Varec Biogas (Ovivo) vs Shand & Jurs for Digestion is rarely a matter of one being objectively better; it is a question of application fit. Varec (Ovivo) brings a large installed base and an integrated approach to the entire gas train, making it a standard choice for general municipal applications where simplicity is valued. Shand & Jurs distinguishes itself in applications demanding the highest sealing efficiency and freeze protection, often preferred in rigorous industrial environments or plants with strict emissions mandates.

For the specifying engineer, the priority must be accurate sizing across the whole relief path — valve, arrester, and emergency device — with density correction and clear material definitions. For the operator, success depends on rigorous adherence to inspection schedules. By understanding the mechanical nuances of these two suppliers, utilities can ensure the long-term safety and efficiency of their anaerobic digestion assets.