Reservoir Design for Hydraulic Power Units

Understanding Hydraulic Reservoir Design Fundamentals

The hydraulic reservoir, often referred to as the hydraulic tank, serves as the lifeblood of any hydraulic power unit (HPU). While it may appear to be simply a container for hydraulic fluid, the reservoir performs multiple critical functions that directly impact system performance, longevity, and reliability. For industrial operations across Nova Scotia and the broader Atlantic Canada region, where equipment often operates in demanding conditions ranging from fish processing plants to forestry operations, proper reservoir design is essential for minimising downtime and maximising productivity.

A well-designed hydraulic reservoir accomplishes several key objectives: it stores an adequate volume of hydraulic fluid, dissipates heat generated during system operation, allows entrained air to escape from the fluid, permits contaminants and water to settle out, and provides a mounting surface for pumps, motors, and other components. Understanding these functions and how they influence design decisions is crucial for engineers and technical managers responsible for specifying or maintaining hydraulic systems.

Reservoir Sizing and Capacity Calculations

Determining the appropriate reservoir size is one of the most fundamental aspects of hydraulic power unit design. The conventional rule of thumb suggests that reservoir capacity should be three to five times the pump flow rate in litres per minute (LPM) for industrial applications. However, this guideline requires careful consideration based on specific operational requirements.

For a hydraulic system with a pump delivering 100 LPM, the traditional approach would specify a reservoir capacity between 300 and 500 litres. This sizing provides adequate fluid residence time for heat dissipation and air separation. In Atlantic Canada's industrial environments, where ambient temperatures can vary significantly between seasons—from -25°C in winter to 30°C in summer—proper sizing becomes even more critical for thermal management.

Factors Influencing Reservoir Capacity

• Duty cycle: Continuous operation systems require larger reservoirs for improved heat dissipation, while intermittent duty applications may function adequately with smaller tanks

• Cylinder displacement: Systems with large cylinders must account for fluid volume changes as cylinders extend and retract

• Accumulator systems: Applications using accumulators need additional reservoir capacity to accommodate stored fluid when accumulators discharge

• Heat load: Higher system pressures and flow rates generate more heat, necessitating larger reservoirs or supplementary cooling

• Available space: Mobile equipment and compact industrial installations may require creative solutions to achieve adequate capacity within spatial constraints

When space limitations prevent achieving the ideal three-to-five times ratio, engineers must compensate through enhanced cooling systems, improved baffle designs, or more efficient fluid conditioning. Many Maritime industrial facilities, particularly those in older buildings or on marine vessels, face these spatial challenges regularly.

Reservoir Construction Materials and Configurations

The selection of reservoir construction materials significantly impacts system performance, maintenance requirements, and operational lifespan. Each material option presents distinct advantages and considerations for specific applications.

Steel Reservoirs

Carbon steel remains the most common choice for industrial hydraulic reservoirs due to its excellent strength-to-cost ratio and ease of fabrication. Reservoirs constructed from steel plate, typically ranging from 6 mm to 12 mm thickness depending on size, provide robust service in demanding environments. For Nova Scotia's industrial sector, where exposure to salt air and humid conditions is common, proper surface preparation and coating systems are essential. Interior surfaces should receive appropriate treatment—either sandblasting followed by compatible primer and topcoat, or specialised epoxy coatings designed for hydraulic fluid compatibility.

Stainless Steel Reservoirs

Applications requiring superior corrosion resistance or operating in food-grade environments benefit from stainless steel construction. Grade 304 stainless steel suits most general applications, while Grade 316 provides enhanced resistance to chloride-induced corrosion—particularly relevant for facilities near the Atlantic coastline or those processing seafood. Although stainless steel reservoirs command a premium of 40-60% over carbon steel alternatives, the reduced maintenance and extended service life often justify the investment for critical applications.

Aluminium Reservoirs

Mobile hydraulic applications and weight-sensitive installations frequently employ aluminium reservoirs. Offering approximately one-third the weight of steel for equivalent volume, aluminium provides excellent thermal conductivity for improved heat dissipation. However, designers must ensure fluid compatibility, as some hydraulic fluid additives can react adversely with aluminium surfaces.

Polyethylene and Composite Reservoirs

Rotomoulded polyethylene reservoirs have gained acceptance for smaller hydraulic power units, typically up to 200 litres capacity. These reservoirs offer inherent corrosion resistance, reduced weight, and lower cost for appropriate applications. However, they present limitations regarding operating temperature ranges and compatibility with certain synthetic hydraulic fluids.

Internal Components and Baffle Design

The internal configuration of a hydraulic reservoir dramatically influences its effectiveness in conditioning fluid and maintaining system performance. Properly designed internal components ensure adequate separation of return flow from suction areas, promote air release, and facilitate contaminant settling.

Baffle Plate Configuration

Baffle plates serve as the primary means of directing fluid flow within the reservoir and separating the return and suction zones. Effective baffle design should extend from the reservoir floor to approximately two-thirds of the fluid level height, creating a tortuous path that maximises fluid residence time. For reservoirs serving high-flow systems, multiple baffles arranged in a serpentine pattern can significantly improve conditioning effectiveness.

The baffle plate should include strategically positioned openings—typically at the bottom to allow settled contaminants to distribute evenly and prevent accumulation in stagnant zones. These openings, usually comprising 10-15% of the baffle area, maintain equalised fluid levels while still providing effective separation.

Return Line Diffusers

Return flow entering the reservoir at high velocity creates turbulence that entrains air and disturbs settled contaminants. Diffuser assemblies spread return flow across a larger area, reducing fluid velocity and minimising these detrimental effects. A properly designed diffuser should reduce return flow velocity to less than 0.3 metres per second at the point of entry into the main reservoir volume.

Suction Strainers and Inlet Configurations

The pump suction line configuration requires careful attention to prevent cavitation and ensure adequate flow. Suction strainers, typically rated at 100-150 mesh (100-150 microns), protect pumps from gross contamination while presenting minimal flow restriction. The suction inlet should be positioned at least 75 mm above the reservoir floor and 75 mm below the minimum operating fluid level to prevent air ingestion and sediment pickup.

Thermal Management and Heat Dissipation

Hydraulic systems convert mechanical energy to fluid power, but inefficiencies throughout the system generate heat that must be effectively dissipated. The reservoir plays a central role in thermal management, and proper design ensures fluid temperatures remain within acceptable operating ranges—typically 40-55°C for optimum fluid viscosity and component longevity.

Heat Generation Sources

Understanding heat sources helps engineers properly size thermal management systems. Primary heat generation occurs through pump inefficiency (typically 10-20% of input power), valve throttling losses, pressure relief operations, and mechanical friction in actuators. A hydraulic system operating at 50 kW with overall efficiency of 70% generates approximately 15 kW of heat that must be dissipated.

Reservoir Heat Dissipation Capacity

Steel reservoir surfaces dissipate heat through natural convection at rates of approximately 6-8 watts per square metre per degree Celsius temperature differential between fluid and ambient air. For a 500-litre reservoir with roughly 4 square metres of exposed surface area, natural dissipation might handle 500-700 watts under typical conditions—often insufficient for continuous-duty industrial systems.

Supplementary Cooling Systems

When reservoir surface area proves inadequate for heat dissipation, supplementary cooling becomes necessary. Options include:

• Air-cooled heat exchangers: Compact and requiring only electrical power, these units suit most industrial installations and provide cooling capacities from 5 kW to over 100 kW

• Water-cooled heat exchangers: Shell-and-tube or plate-type exchangers offer superior cooling efficiency where process water is available—common in Nova Scotia's manufacturing facilities with existing cooling water infrastructure

• Reservoir cooling coils: Internal or external cooling coils provide a cost-effective solution for moderate cooling requirements, utilising facility water or glycol systems

For operations in Atlantic Canada, seasonal temperature variations must factor into cooling system design. Systems that operate comfortably during winter months may require substantial cooling capacity during summer peak temperatures.

Accessories and Instrumentation

Properly equipped reservoirs include essential accessories that facilitate operation, maintenance, and system monitoring. These components, while sometimes overlooked, significantly impact system reliability and serviceability.

Breather and Filler Assemblies

As fluid levels fluctuate during system operation, air must enter and exit the reservoir. Breather assemblies filter incoming air to prevent atmospheric contaminants from entering the system. For industrial environments, breathers with 3-micron filtration and desiccant cartridges maintain fluid cleanliness and remove moisture—particularly important in Maritime climates where humidity levels often exceed 80%.

Filler openings should incorporate screens (typically 100 mesh) to prevent inadvertent contamination during fluid additions. Combined filler-breather assemblies offer convenience and reduced penetrations through the reservoir top plate.

Level Indicators and Switches

Visual level indicators provide operators with immediate feedback on fluid condition and quantity. Sight glasses or level gauges should span the full operating range, clearly marking minimum and maximum levels. For critical systems, level switches provide electrical signals for low-level alarms or automatic shutdown, preventing pump damage from fluid starvation.

Temperature Monitoring

Temperature gauges or transmitters mounted in the reservoir provide essential operational data. Dial thermometers suit basic applications, while electronic transmitters enable integration with plant control systems and trending analysis. Temperature monitoring points should be positioned away from return lines to measure representative bulk fluid temperature.

Drain and Clean-out Provisions

Reservoir bottoms should incorporate drain connections sized for complete fluid evacuation within reasonable timeframes—typically 25 mm to 50 mm diameter for industrial reservoirs. Sloped bottoms (minimum 1:100 grade) ensure complete drainage toward the outlet. Clean-out covers, either bolted plates or threaded access ports, facilitate interior inspection and cleaning during maintenance intervals.

Design Standards and Best Practices

Hydraulic reservoir design should align with recognised industry standards that establish minimum requirements for safe, reliable operation. The Joint Industry Council (JIC) specifications, National Fluid Power Association (NFPA) standards, and ISO guidelines provide frameworks for reservoir design.

Key Design Standards

• NFPA T3.16.2: Establishes reservoir design criteria including sizing, construction, and component specifications

• ISO 4413: Provides general rules and safety requirements for hydraulic systems, including reservoir considerations

• CSA B51: Canadian standard applicable to pressure-containing components that may affect reservoir-mounted equipment

Maritime-Specific Considerations

Hydraulic systems operating in Nova Scotia and Atlantic Canada face unique environmental challenges. Salt-laden air accelerates corrosion on external surfaces and can contaminate fluid through breathers. Seasonal temperature extremes require attention to fluid viscosity selection and thermal management. Many regional industries—fishing, forestry, mining, and marine operations—subject hydraulic equipment to vibration, shock loading, and remote operating locations where maintenance access may be limited.

Designing reservoirs for these conditions requires enhanced corrosion protection, robust construction, and careful attention to serviceability. Specifying appropriate fluid types, implementing effective contamination control, and providing adequate thermal management addresses these regional challenges effectively.

Partner with Sangster Engineering for Your Hydraulic System Needs

Proper hydraulic reservoir design requires careful analysis of system requirements, operating conditions, and application-specific factors. Whether you're specifying a new hydraulic power unit, upgrading existing equipment, or troubleshooting performance issues, the engineering team at Sangster Engineering Ltd. brings decades of experience to every project.

Based in Amherst, Nova Scotia, our firm serves clients throughout Atlantic Canada with comprehensive mechanical engineering services, including hydraulic system design, analysis, and optimisation. We understand the unique challenges facing Maritime industries and deliver practical, cost-effective solutions tailored to your specific requirements.

Contact Sangster Engineering Ltd. today to discuss your hydraulic system requirements. Our professional engineers are ready to help you achieve reliable, efficient hydraulic power unit performance through expert reservoir design and system integration. Let us put our regional expertise and technical knowledge to work for your organisation.

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.