Smith & Loveless vs Shand & Jurs for Digestion

Introduction

The anaerobic digester gallery is arguably the most complex and hazardous environment within a Water Resource Recovery Facility (WRRF). Engineers are tasked with balancing two volatile forces: the biological/hydraulic requirement to keep sludge moving and heated, and the thermodynamic requirement to safely contain and manage explosive methane gas. A single specification error in either the hydraulic loop or the gas train can lead to catastrophic seal failure, souring of the biological process, or, in worst-case scenarios, structural rupture.

When engineering teams approach the specification of these systems, the conversation often turns to industry stalwarts. The comparison of Smith & Loveless vs Shand & Jurs for Digestion is less about choosing one vendor over the other for a single component, and more about understanding the interface between two distinct engineering disciplines: high-solids hydraulic handling (the domain of Smith & Loveless) and low-pressure gas safety management (the domain of Shand & Jurs). While they occupy different physical spaces on the P&ID, their interaction is critical. The pump vibration from a sludge loop can trip sensitive gas instrumentation, and improper gas pressure relief can impact the hydraulic head available for recirculation.

This article serves as a technical guide for consulting engineers and utility directors. It moves beyond the “glossy brochure” features to analyze the engineering constraints, selection criteria, and operational realities of integrating these major equipment categories. By understanding the specific strengths and specification requirements of Smith & Loveless (S&L) alongside Shand & Jurs (S&J), engineers can design digester complexes that optimize both hydraulic efficiency and process safety.

How to Select / Specify

Successful digester design relies on the correct application of equipment to specific process zones. When evaluating Smith & Loveless vs Shand & Jurs for Digestion, the engineer is essentially specifying the “heart” (pumps) and the “lungs” (gas safety) of the system. The selection criteria differ vastly between the liquid and gas phases.

Duty Conditions & Operating Envelope

The operating envelope for digestion equipment is defined by extremes—high solids on the liquid side and extremely low pressures on the gas side.

• Hydraulic Duty (Smith & Loveless Context): For sludge transfer and recirculation, the primary variable is solids concentration (typically 3% to 6% TS, but potentially higher in thermal hydrolysis applications). Engineers must specify pumps capable of passing 3-inch spheres minimum. The operating point must be selected to maintain scouring velocity (typically >3 ft/s) in piping to prevent grease accumulation, without shearing the floc excessively if downstream dewatering is sensitive to particle size.
• Pneumatic Duty (Shand & Jurs Context): Gas safety equipment operates in “Inches of Water Column” (WC), not PSI. A typical operating range is 6″ WC to 12″ WC. The “Set Point” versus “Full Flow” pressure is critical. Engineers must calculate the Maximum Expected Operating Pressure (MEOP) and ensure the Pressure Vacuum Relief Valve (PVRV) is set usually 10% above MEOP but below the structural design pressure of the tank or cover.

Materials & Compatibility

Corrosion is the universal enemy in digester galleries, but the chemical attack vectors differ between the liquid and gas phases.

• Liquid Phase Materials: Sludge pumps typically require high-chrome iron or hardened ductile iron volutes and impellers to resist the abrasion of grit that bypasses headworks. S&L specifications often focus on heavy wall thickness to extend life against abrasion.

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• Gas Phase Materials: The presence of wet Hydrogen Sulfide (H₂S) creates a highly corrosive sulfuric acid environment upon condensation. For S&J equipment, standard aluminum bodies are often insufficient for high-H₂S applications. Engineers should specify 316 Stainless Steel for valve bodies, pallets, and seats. For flame arresters, the element material is critical; 316SS is the standard minimum to prevent corrosion from fusing the element layers together, which would block gas flow.

Hydraulics & Process Performance

The interaction between fluid movement and gas generation defines the process stability.

For the hydraulic side, the focus is on Non-Clogging Performance. S&L pumps, often used for recirculation or heating loops, must employ impeller geometries (X-Peller or similar mono-port designs) that prevent ragging. If the pump rags, heat transfer stops, and the digester goes sour.

For the gas side, the hydraulic equivalent is Flow Capacity at Overpressure. A common error is sizing a relief valve based on pipe size rather than flow capacity. S&J valves are rated by flow curves (SCFM air) at specific overpressures. The engineer must verify that the valve can vent the maximum possible gas generation rate (plus any emergency gas displacement from sludge pumping) without exceeding the tank’s pressure rating.

Installation Environment & Constructability

Digester galleries are notorious for being cramped, humid, and classified as hazardous locations.

• Hazardous Area Classification: Following NFPA 820, most digester environments are Class 1, Division 1 or 2. S&L pumps must be specified with explosion-proof (XP) motors. S&J equipment is inherently mechanical, but any associated limit switches or proximity sensors for SCADA monitoring must be intrinsically safe or XP rated.
• Freeze Protection: Digester gas is wet. In cold climates, S&J valves require insulation jackets and possibly heat tracing to prevent the valve seat from freezing shut (which turns the digester into a bomb) or freezing open (which vents odors and gas).

Reliability, Redundancy & Failure Modes

Reliability strategies differ between active machinery (pumps) and passive safety devices (valves).

• Active Failure (Pumps): The most common failure mode for sludge pumps is seal failure due to grit or ragging. Specifying double mechanical seals with a flushing water system (or glycol barrier) is standard. Redundancy should be N+1, allowing one pump to be pulled for maintenance without stopping recirculation.
• Passive Failure (Valves): The primary failure mode for gas safety valves is “sticking” due to tar/grease buildup or corrosion. Unlike pumps, there is rarely an “N+1” redundancy for a tank PVRV; it is a single point of failure. Reliability is achieved through “pallets” with Teflon seating diaphragms to prevent sticking and by specifying ease of access for cleaning.

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Maintainability, Safety & Access

Maintenance access is often an afterthought that leads to operational neglect.

For S&L pumps, the “wet well mounted” or “flooded suction” dry pit configurations dominate. The main maintenance task is clearing rags and adjusting wear plate clearances. Designs that allow front-access to the impeller without decoupling the motor (often a feature of specific S&L split-coupled designs) reduce maintenance hours significantly.

For S&J gas equipment, the challenge is height. These valves sit on top of digester covers. Engineers must specify davit arms or hinge mechanisms on the valves themselves to allow a single operator to inspect the valve seat without needing a crane. If a flame arrester is heavy (cast iron), a dedicated lifting lug and removal mechanism must be part of the design.

Lifecycle Cost Drivers

The Total Cost of Ownership (TCO) calculus includes energy, parts, and safety compliance.

• Energy: Pumping sludge accounts for a significant portion of the digester’s parasitic load. High-efficiency motors (IE3/IE4) and VFDs on S&L pumps allow operators to dial in recirculation rates based on volatile solids loading, saving energy.
• Compliance Costs: Gas safety equipment requires regular testing to verify set points. S&J offers “expander” technologies or on-site testing kits. The cost of failing a safety inspection or an uncontrolled venting event (fines) far outweighs the CAPEX of premium valve materials.

Comparison Tables

The following tables break down the distinctions between the hydraulic focus (Smith & Loveless) and the gas safety focus (Shand & Jurs). Use these matrices to ensure all subsystems of the digester are covered in the specification.

Engineer & Operator Field Notes

Real-world experience often diverges from the theoretical design. The following notes are compiled from field observations regarding the installation and operation of equipment typical to these two manufacturers within a digestion context.

Commissioning & Acceptance Testing

Commissioning a digester is a sequenced event. The liquid side is tested first (hydrostatic testing of the tank), followed by the gas side (pneumatic testing).

• S&L Pump Commissioning: Standard factory pump curves are generated with clear water. In the field, sludge viscosity affects head and efficiency. Engineers should perform a draw-down test using water during startup to establish a baseline. Verify vibration levels are within Hydraulic Institute standards before sludge is introduced.
• S&J Valve Commissioning: DO NOT rely solely on the factory certificate. Shipping vibrations can shift weights or damage pallets. A “bench test” or “pop test” should be performed on site before the valves are bolted to the digester cover. Ensure the shipping hold-downs (often red zip ties or blocks) are removed from the valve pallets. A surprisingly common failure is leaving shipping blocks in place, rendering the safety valve useless.

Common Specification Mistakes

Over-Specifying Pump Head (S&L): Engineers often add excessive safety factors to friction loss calculations for sludge lines. If an S&L pump is sized for 60ft TDH but the system only requires 30ft, the pump will run to the right of its curve, leading to cavitation, vibration, and premature bearing failure. Use VFDs to trim the pump to the actual system curve.

Under-Specifying Pressure Drop (S&J): A flame arrester creates significant pressure drop. If the engineer selects a flame arrester based on line size (e.g., 6-inch pipe = 6-inch arrester) without checking the flow vs. pressure drop curve, the gas system may have too much backpressure. This can cause gas to blow the PVRV on the tank roof instead of traveling to the boiler or flare. Always check the “Delta P” across the entire S&J gas train.

O&M Burden & Strategy

Operational strategies for these two equipment classes are distinct:

• Liquid Train (S&L): Maintenance is condition-based. Use vibration analysis and amp draw monitoring. A spike in amps usually indicates a partial clog; a drop in amps often indicates air binding or worn wear rings.
• Gas Train (S&J): Maintenance is interval-based. Gas valves do not “signal” when they are failing; they simply stick. A mandatory Preventive Maintenance (PM) schedule (every 6 months) to clean valve seats with a non-sparking solvent is critical. Operators must also drain condensate drip traps daily—liquid accumulation in gas lines is the #1 cause of pressure fluctuation.

Troubleshooting Guide

Symptom: Digester Pressure Spikes unexpectedly.
Check the Liquid Side (S&L): Is the sludge feed pump running too fast? A sudden influx of sludge displaces gas volume. If the gas piping is undersized, pressure spikes.
Check the Gas Side (S&J): Is the flame arrester bank clogged with sulfur/particulate? A clogged arrester blocks flow to the flare, causing pressure to back up to the tank PVRV.

Design Details / Calculations

Integrating Smith & Loveless vs Shand & Jurs for Digestion requires harmonizing hydraulic calculations with gas laws.

Sizing Logic & Methodology

1. Estimating Gas Production for Valve Sizing

To size S&J equipment, you must estimate the Peak Gas Production.
Rule of Thumb: 15 – 18 cu. ft. of gas per pound of Volatile Solids (VS) destroyed.
Calculation: Q_gas = (Sludge Flow × %Solids × %Volatile × %Destruction × 16) / 1440 min/day.
Add the “Pump In” rate (Q_pump) from the S&L feed pumps (converted to CFM) to this gas production rate to determine the “Normal Venting” requirement.

2. Sizing Sludge Recirculation Pumps

To size S&L pumps for heating/mixing:
Turnover Time: Typically 4 to 6 hours for the entire tank volume.
Flow Rate (Q): Volume / Turnover Time.
TDH Calculation: Friction loss is higher for sludge. Use multipliers on the Hazen-Williams C-factor (often C=100 or lower for sludge lines) or specific rheological models if TS > 5%.

Specification Checklist

When preparing the bid documents, ensure these items are included to cover the interface between these technologies:

• For S&L Pumps:
  • Mechanical Seal Type: Cartridge style, tungsten carbide vs. silicon carbide faces.
  • Passage Size: Minimum sphere size (2.5″ – 3″).
  • Clean-out Ports: Hand-hole access on the volute.
  • VFD Compatibility: Inverter-duty rated motors.

• For S&J Gas Safety:
  • Body Material: low-copper aluminum (std) or 316SS (severe service).
  • Diaphragm Material: PTFE (Teflon) to resist sticking.
  • Proximity Switches: Included on PVRV to alarm SCADA when valve opens.
  • Flame Element: Modular/removable bank for easy cleaning.

Standards & Compliance

Adherence to standards is mandatory for insurance and safety:

• NFPA 820: Standard for Fire Protection in Wastewater Treatment and Collection Facilities. Defines the hazardous zones around the digester which dictates S&L motor ratings and S&J instrument ratings.
• API 2000: Venting Atmospheric and Low-Pressure Storage Tanks. The governing standard for sizing S&J relief valves.
• Ten States Standards: Provides minimum redundancy and capacity requirements for sludge pumps and gas handling systems.

Frequently Asked Questions

What is the primary difference between specifying Smith & Loveless vs Shand & Jurs for Digestion projects?

The primary difference is the medium handled and the physics involved. Smith & Loveless is typically specified for the liquid/sludge hydraulic loops (recirculation, transfer, heating), requiring non-clog centrifugal pump technology. Shand & Jurs is specified for the gas safety train (pressure relief, flame arresters, flares), requiring precise low-pressure pneumatic control. They are complementary portions of the same project, not interchangeable equipment.

How do I calculate the required capacity for a Shand & Jurs pressure relief valve?

Capacity is calculated based on API 2000 standards. It involves summing the maximum gas generation rate (biological) and the maximum liquid fill rate (mechanical pumping displacement). If you have two 500 GPM Smith & Loveless sludge pumps that could run simultaneously, you must convert 1000 GPM to CFM (approx 133 CFM) and add this to the biological gas production peak to determine the minimum venting capacity required to prevent tank over-pressurization.

Can Smith & Loveless pumps handle the high temperatures of digester recirculation?

Yes, but it must be specified. Standard wastewater pumps are often rated for 104°F (40°C). Digester recirculation can reach 140°F (60°C) or higher in thermophilic processes. You must specify high-temperature elastomers (Viton/FKM) and ensure the motor insulation class (Class F or H) is suitable for the ambient temperature in the gallery plus the heat rise from the fluid.

Why do Shand & Jurs flame arresters clog frequently?

Flame arresters function by having very small flow passages to quench a flame front. These small passages act as a filter for impurities in the biogas, such as siloxanes, particulate matter, and sulfur condensates. Frequent clogging is often a sign of poor upstream gas scrubbing or insufficient condensate removal. Using a “corrugated” element rather than a “ribbon” element can sometimes improve run times, but regular cleaning is unavoidable.

What happens if the Smith & Loveless recirculation pump fails?

If sludge recirculation stops, the digester contents will stratify. Grit will settle to the bottom (reducing active volume), and a scum blanket will form on top (potentially blocking gas release). This leads to “sour” digesters where pH drops and gas production halts. This is why N+1 redundancy on recirculation pumps is standard engineering practice.

How does “set pressure” differ from “relieving pressure” in gas valves?

The “set pressure” is when the valve starts to open (leak). The “relieving pressure” (or overpressure) is the pressure at which the valve is fully open and passing its rated flow. For Shand & Jurs valves, if the tank design pressure is 12″ WC, you might set the valve at 10″ WC, but you must verify that at full flow, the pressure build-up doesn’t exceed 12″ WC. This “pressure accumulation” is a critical calculation.

Conclusion

When evaluating Smith & Loveless vs Shand & Jurs for Digestion, the engineer is not choosing a winner, but rather orchestrating a complex system where two specialized technologies must perform in harmony. The success of a digester project depends on the seamless integration of these subsystems. The hydraulic force provided by Smith & Loveless ensures biological mixing and heating, while the pneumatic precision of Shand & Jurs protects the physical assets and personnel from explosive hazards.

For the specifying engineer, the goal is to write a specification that respects the distinct duty cycles of both fluid and gas. By ensuring that pump capacities are matched with relief valve sizing, and that materials are selected for the specific corrosive nature of each phase, utilities can achieve a digester lifecycle that minimizes downtime and maximizes safety. Always consult the specific pump curves and valve flow certifications for the exact models being considered, as generic assumptions in the digester gallery can lead to costly operational challenges.