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 gas safety equipment has largely been a binary choice in the North American market. 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. While both manufacturers are industry standards, the nuances in their Pressure Vacuum Relief Valve (PVRV) designs, flame arrester technologies, and waste gas burner configurations can lead to significant differences in operational expenditure (OPEX) and site safety profiles.

Digester covers—whether floating, fixed, or membrane—operate within extremely narrow pressure bands, often measured in inches of water column (w.c.). A failure in the relief system can lead to catastrophic structural failure (cover blow-off) or implosion due to 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 detailed, neutral engineering analysis of the equipment offered by these two primary 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.

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 (e.g., < 10%), valves may "simmer," leading to fugitive emissions and accelerated seat wear.
• Flow Capacity (SCFH): Sizing must account for both normal venting (gas production + pump-in displacement) and emergency venting (fire exposure). Both Varec and Shand & Jurs offer capacity curves, but these must be de-rated for the specific specific gravity of biogas (typically 0.86-0.90 relative to air).
• Vacuum Requirements: Vacuum relief is often the more critical scenario to prevent tank implosion during rapid sludge withdrawal. The valve’s flow characteristics at low vacuum settings (e.g., -2 inches w.c.) 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 “sweet” gas, but “sour” gas requires upgrades.

• Body Materials: Low-copper aluminum is the industry standard for housing. 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 (Teflon) and FEP are common for their chemical resistance and non-stick properties, crucial 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, but high-performance valves (from both Varec and Shand & Jurs) can exceed these standards.
• 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 (weight-loaded vs. pilot-operated).

Installation Environment & Constructability

Where the valve is placed impacts its selection.

• 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 will increase the effective opening pressure.
• Winterization: In northern climates, freezing condensate is a primary failure mode. Insulation jackets and steam/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 Limits: 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 (1 duty / 1 standby) or to have a separate emergency relief manway. This allows one unit to be isolated for maintenance without taking the digester offline.

Controls & Automation Interfaces

Modern plants increasingly require monitoring of mechanical relief devices.

• 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 (U-tube or Magnehelic) integrated into the valve body or adjacent piping is essential for operator rounds.

Maintainability, Safety & Access

If a valve cannot be easily maintained, it will fail.

• 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 major Tier 1 manufacturers for standard specs.
• Maintenance Labor: The primary OPEX driver. Valves that require monthly cleaning vs. quarterly cleaning significantly change the lifecycle cost.
• Gas Loss: Leaky valves result in lost energy potential (less gas for cogen). Premium sealing technologies pay for themselves through increased methane capture.

Comparison Tables

The following tables provide a side-by-side analysis of the two primary technologies typically evaluated. Table 1 compares the manufacturers’ general approach to the digestion market. Table 2 provides a technical feature matrix for their flagship Pressure Vacuum Relief Valves (PVRV), the most critical safety component.

Table 1: Manufacturer Profile & Market Position

Manufacturer	Primary Strengths	Typical Applications	Limitations/Considerations	Maintenance Profile
Varec Biogas (Ovivo) Series 5800 / 5810 / 5820	– Massive install base (legacy standard) – Modular, simple weight-loaded design – Integrated “system” approach (Ovivo covers + gas train) – Robust cast aluminum housings	– Municipal Anaerobic Digesters – Waste Gas Burner Systems – Retrofits of existing Varec installations	– Standard pallet design can be prone to sticking if not cleaned regularly – “Air cushion” pallet designs rely on clean seating surfaces	– Moderate: Requires regular inspection for struvite/condensate. – Parts are widely available but OEM parts can be premium priced.
Shand & Jurs (L&J Tech) Model 94020 / Expanda-Seal	– “Expanda-Seal” technology offers superior tightness near set pressure – Insulation jackets are often cited as industry benchmark – High resistance to seat freezing	– High-performance Digesters – Cold climate installations – Applications requiring minimized gas loss	– Diaphragm materials must be carefully selected for chemical compatibility – Slightly more complex internal mechanism than simple pallet weights	– Low to Moderate: “Expanda-Seal” design reduces simmer and fouling. – Diaphragms eventually require replacement (wear item).

Table 2: Technical Feature Matrix – Weight Loaded PVRVs

Feature	Varec Biogas (Typical)	Shand & Jurs (Typical)	Engineering Implication
Seating Technology	Standard O-Ring or Teflon Insert Pallet	Expanda-Seal (Inflating Diaphragm)	S&J design typically allows operating pressure closer to set pressure (90%+) without leakage. Varec design is robust but may simmer earlier.
Winterization	Heat Tracing / Insulation Jacket	“All-Weather” Coating + Insulation Jackets	Both effective, but insulation jacket fit and finish is critical. S&J insulation design specifically targets minimizing thermal bridging.
Material Options	Aluminum, 316SS, Ductile Iron	Aluminum, 316SS, Cast Iron, Hastelloy	Both cover standard municipal needs. S&J offers broader exotic alloys for industrial/petrochem crossover.
Pallet Guides	Center Stem / Side Guide	Center Stem / Poly-FEP Guides	Guide friction causes hysteresis. Both use low-friction sleeves (Teflon/FEP) to prevent sticking.
Maintenance Access	Removable Hoods/Covers	Quick-Clamp Access (on some models)	Ease of access encourages operator compliance. Bolted covers take longer to service than clamped designs.

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 SAT (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 utilize shipping blocks or cardboard 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 soapy water or leak detection fluid on the flange faces and the valve outlet to check for premature lifting.

Common Mistake: Neglecting to account for the static head of the gas column in “pipe-away” applications. If a relief valve is mounted 20 feet below the discharge point, condensate accumulation in the discharge pipe can add backpressure, effectively raising the set pressure and endangering the tank. Always install drip traps at low points.

Common Specification Mistakes

• Specifying “Air” Capacity: Engineers sometimes specify flow rates in SCFM of Air without correcting for Biogas density. While conservative (since biogas is lighter), it 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 max flow, the tank is unprotected.
• Material Mismatch: Specifying aluminum internals for a system with high H2S (>500 ppm) 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 “glues” the pallet to the seat.
• Maintenance Interval:
  • Monthly: Visual inspection of desiccants (if applicable) and check for icing in winter.
  • Quarterly: Exercise the valve (manually lift) to ensure freedom of movement.
  • Annually: Full tear-down, cleaning of seats with non-abrasive solvent, and inspection of diaphragms/O-rings.
• Spare Parts: Keep a full set of soft goods (O-rings, diaphragms, gaskets) and at least one spare pallet assembly on the shelf. Lead times for these OEM parts can be 4-8 weeks.

Troubleshooting Guide

• Valve “Chatter” (Rapid opening/closing): usually indicates the valve is oversized for the flow rate, or the inlet piping is too restrictive (inlet pressure loss > 3% of set pressure).
• Continuous Leakage: Check for debris on the seat. If the seat is clean, check if the operating pressure is too close to the set pressure (Simmering).
• Sticky Operation: Likely caused by siloxane buildup. Requires solvent cleaning. Do not use metal scrapers on Teflon 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 ($Q_{total}$) is the sum of two components:

1. Normal Venting:
   • Liquid Movement: Maximum pump-in rate ($Q_{pump}$) converted to gas displacement. Typically 1 GPM $approx$ 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 ($A$).
     Rule of Thumb: For many municipal digesters, the emergency venting requirement dwarfs the normal venting requirement.

Pro Tip: When sizing for Varec Biogas (Ovivo) vs Shand & Jurs for Digestion, request flow curves at 10% overpressure specifically. Many standard industrial curves show flow at 100% overpressure, which is often above the structural failure point of a concrete digester lid.

Specification Checklist

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

• [ ] Fluid Service: Anaerobic Digester Gas (give composition: %CH4, %CO2, ppm H2S, Moisture content).
• [ ] Set Pressure & Vacuum: Defined in inches w.c. or mbar.
• [ ] Maximum Allowable Working Pressure (MAWP): of the tank.
• [ ] Leakage Class: e.g., “Less than 0.5 SCFH at 90% of set pressure.”
• [ ] Certifications: CSA/UL approval for electrical components (switches/trace heating) in Class 1 Div 1 areas.
• [ ] Testing: Requirement for Certified Test Reports (CTR) for leakage and set pressure.

Standards & Compliance

• API 2000 (7th Ed): “Venting Atmospheric and Low-Pressure Storage Tanks.” The bible for sizing.
• 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 if flame arresters are integrated into the relief assembly (common in compact designs).

FAQ Section

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

The primary technical difference lies in the sealing technology. Shand & Jurs is renowned for its “Expanda-Seal” diaphragm design, which uses the internal tank pressure to inflate the seal against the pallet, creating a tighter seal closer to the set point. Varec typically utilizes a weight-loaded pallet with an O-ring or soft insert. Varec’s design is simpler and extremely robust, while Shand & Jurs’ design offers superior 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. You must calculate the required venting capacity for liquid pump-in/pump-out, thermal expansion/contraction, biological gas production, and emergency fire relief. Once the total SCFH is determined, you select a valve size (e.g., 4″, 6″, 8″) that can pass that flow at a pressure typically no higher than 10-20% above the set pressure, ensuring the tank’s structural limit is not breached.

Why do digester relief valves fail?

The most common causes of failure are sticking due to “cementing” (struvite or frozen condensate) and corrosion. Biogas contains moisture and hydrogen sulfide; if the valve body isn’t heated/insulated in winter, ice forms. If the materials aren’t 316SS or aluminum, H2S corrodes them. Lack of maintenance (cleaning valve seats) leads to leakage or the valve failing to open at the set pressure.

What is the typical lifespan of these valves?

With proper maintenance, the cast body housings of both Varec and Shand & Jurs valves can last 20+ years. However, the 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 the severity of the H2S exposure.

Should I use a flame arrester with my PVRV?

Yes, if the PVRV vents to the atmosphere where an external ignition source (lightning, maintenance work, static) could ignite the gas plume. The flame arrester prevents the flame front from traveling back into the digester. However, the flame arrester adds pressure drop and requires frequent cleaning to prevent clogging, which can dangerously block the relief path.

How much do Varec vs Shand & Jurs valves cost?

Costs are generally comparable for standard municipal specifications. A typical 6-inch assembly might range from $4,000 to $8,000 depending on materials (aluminum vs stainless) and accessories (insulation jackets, switches). While base prices are similar, decision-makers should evaluate the “Total Cost of Ownership” based on maintenance frequency and the cost of replacement parts (diaphragms vs O-rings).

Conclusion

Key Takeaways

• Safety First: The PVRV is the final line of defense against tank structural failure. Never compromise on sizing verification (API 2000).
• Sealing Tech Matters: Shand & Jurs “Expanda-Seal” offers tighter sealing near set pressure (good for maximizing gas capture), while Varec offers simpler, robust mechanics (good for ease of maintenance).
• Material Selection: Always specify 316SS trim/pallets for biogas service. Aluminum bodies are acceptable if H2S is low/moderate.
• Winterization: In freezing climates, insulation jackets and trace heating are not optional; they are critical for reliability.
• Maintenance: Neither option is “maintenance-free.” Budget for semi-annual cleaning to remove struvite and siloxanes.

The choice between Varec Biogas (Ovivo) vs Shand & Jurs for Digestion is rarely a matter of one being objectively “better” than the other; rather, it is a question of application fit. Varec (Ovivo) brings the strength of a massive install base and a highly integrated approach to the entire gas train, making them a safe, standard choice for general municipal applications, especially 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 hydraulic modeling (accounting for specific gravity and overpressure limits) and clear material definitions. For the operator, success depends on rigorous adherence to inspection schedules. By understanding the mechanical nuances of these two industry leaders, utilities can ensure the long-term safety and efficiency of their anaerobic digestion assets.

source https://www.waterandwastewater.com/varec-biogas-ovivo-vs-shand-jurs-for-digestion/