The Ultimate Guide : Hydraulic Tank

Hydraulic Tank also commonly known as reservoir serve as the storage for hydraulic oil. If properly designed it also function as conditioning devices, and if not properly sized it will breakdown entire hydraulic system as cavitation, contamination problems may occur. This article present all fundamentals from basics to advance to properly design and size the reservoir correctly.

Before we dive in the nitty gritty of hydraulic tank, let’s tick mark the basics of hydraulic tank.

Industrial Hydraulic Tank and Mobile Hydraulic Tank

Compare to industrial tank, mobile hydraulic tank are prone to extreme and often adverse condition. As mobile hydraulic tank have to be installed in a limited available space. So they require custom design consideration. Application includes, but not limited to heavy terrain forklift, excavators, backhoe loader, hay bailers as well as military applications.

• Storing Hydraulic Oil
• Providing source of oil to hydraulic system
• Tank Walls acting as a heat exchanger to dissipate heat
• Preparing the oil for next working cycle

Hydraulic reservoirs in mobile equipment face unique challenges due to the constant movement of the machinery and the wide range of environmental temperatures they must endure.

Yet, one of the most critical hurdles is the constraint on available space.

Size and weight limitations require equipment to operate with tanks that typically require custom design, such that it fits into the exact space available with meeting the technical requirements.

Custom-built hydraulic reservoirs offer a distinct edge over pre-manufactured options, as they can be specifically tailored to match the space and technical demands required for peak efficiency and performance.

Through the use of 3D modeling, user can optimize the reservoir’s structure, ensuring the best possible balance between cost, size, and weight without sacrificing functionality or durability.

1. Utilize regulated air from the machine’s pneumatic system, if available. This method is generally the most efficient.
2. Trap air within the reservoir’s clearance volume (above the fluid) and allow the thermal expansion of the fluid to compress the air, creating pressure within the reservoir.
   A pressure cap (breather) helps to maintain the desired pressure inside the tank while also releasing excess pressure, when retracting cylinders return more oil to the reservoir than the pumps need. The cap should have low-pressure overshoot capabilities.
3. Source pressurized air from the scavenge pump of a two-cycle diesel engine.

When a reservoir is pressurized, it is essential to calculate the stresses exerted on its walls. (For how wall stress could affect the walls of tank, consider checking out this Video from LunchBox Session)

Large flat surfaces in reservoirs can quickly experience high stresses, leading to significant deflections due to the fluid’s weight.

When adding peripheral equipment like ladders or air tanks to reservoirs, it’s important to evaluate the plate thickness and the potential need for stiffeners.
In Fig.-1, ladders are welded with hydraulic tank.

Strategically positioning baffle plates can offer the necessary support while avoiding the extra cost and weight of additional reinforcements.

Baffles plates used to direct, contain, calm or even cool the fluid returned to tank. And to help cool return fluid, it should be directed toward the outer walls of the reservoir.

Fluid should not flow between different levels within the tank. This issue can arise due to uneven fluid levels between areas separated by a poorly designed baffle or a multi-level tank bottom.

Baffles should be designed:

• for strength
• to distribute oil flow throughout the tank
• to create a separation between suction and return
• to incorporate a small slot in the baffle(s) to balance oil volume each side (baffle plate design should avoid creating dead spots where fluid could unintentionally accumulate.)
• to lengthen oil dwell time (i.e. the time the oil takes to move from return to suction, to better cool the oil and help remove air bubbles from the oil)

When sizing baffles, return diffusers, or filters in the return portion of the hydraulic tank, it is crucial to account for the maximum return flow rate, which often exceeds the pump’s output when cylinder with the largest area ratio (difference between the cylinder’s cap end and head end areas) is being retracted at full speed.
The return flow will surpass the pump’s delivery rate. This happens because fluid returning from the cylinder’s larger side combines with the flow of the retracting smaller side, creating a return flow greater than the flow into the cylinder. Design of baffle should be such that the return flow never aimed at a fluid intake line.

To ensure efficient and safe operation, baffles and other components in the return path should be sized to manage this peak return flow. Return diffusers and filters should be selected with sufficient capacity to prevent excessive backpressure and to ensure proper flow distribution within the tank.

In mobile equipment, return line velocities often surpass 10 ft./sec.

To reduce this, it’s recommended to size return manifolds or plumbing such that they bring velocities down to 5 ft./sec. or lower.

Using return line diffusers effectively slows flow rates and helps mix the fluid without causing agitation.

Diffusers can commonly lower velocities to around 1 ft./sec. or less. Installing perforated metal with 40% open area at the ends of return lines is also effective.

Whenever possible, the ends of return lines should be cut at a 45º angle and directed toward the outer wall.

To understand desiccant breathers, first, we need to know what a desiccant is. A desiccant is a hygroscopic material that absorbs moisture to maintain dryness. You’ve likely seen silica gel packets labeled “do not eat” in packaging bottle or shoes. Silica gel is a common desiccant. Other desiccants exist as well.

When moist air enters a reservoir, condensation can form on the interior walls as ambient temperatures decrease.

Desiccant breathers, which are multi-layered devices, are commonly used to block moisture and particles from entering equipment such as gearboxes, pumps, and reservoirs.

These breathers are designed to filter the ambient air drawn into the reservoir.

However, the desiccant material has a limited lifespan, so if the breather isn’t replaced at the recommended intervals, it will eventually allow moisture to pass through unchecked.

To maintain system efficiency and avoid issues such as cavitation, it’s crucial to ensure a smooth transition from the low velocity of the fluid in the reservoir to the pump inlet lines.

Inlet line velocities should not exceed 5 feet per second in large lines.

The pressure drop in a line increases as the diameter decreases due to the greater fluid shear area. This means that for the same flow velocity, smaller lines will experience higher pressure drops than larger lines.

To reduce the risk of vortexing (which can cause cavitation), several techniques can be applied:

1. Cutting the inlet line at an angle and pointing the mouth downward increases the throat area, which helps reduce vortexing and improves fluid flow into the pump.
2. Positioning the inlet tube deeper in the oil increases the fluid pressure around the inlet and reduces the likelihood of vortex formation.
3. Adding vanes to the throat of the tube helps stabilize the flow by extending outward and breaking up vortex formations.

These methods ensure a stable and efficient fluid flow into the pump, reducing the risk of cavitation and extending the pump’s operational life.

Mobile equipment tanks are designed to fit within specific spaces, often leading to challenges with tank volume. Proper tank sizing requires careful consideration of several key calculations.

In the hydraulic system, drawdown refers to the decrease in the fluid level in the hydraulic reservoir as hydraulic components like actuators, cylinders, and motors consume the fluid.

Ensuring that the hydraulic tank has sufficient fluid during maximum drawdown is critical to maintain hydraulic system efficiency, allow proper settling of contaminants, and prevent issues such as cavitation or pump failure.

Key Factors in Drawdown Calculation:

1. Maximum Output: The calculation typically involves determining the maximum hydraulic fluid consumption of the system. This is the fluid volume displaced by the cylinders when they are acting simultaneously.
2. Fluid Capacity During Drawdown: The tank must have enough fluid even when the maximum drawdown occurs. This is essential to avoid the tank running dry, which could lead to severe damage to the hydraulic pump.
3. Residence Time: Residence time (Dwell time) is the amount of time the fluid remains in the reservoir. The longer the residence time, the more opportunity there is for contaminants like air, water, and solid particles to settle out before the fluid is recirculated into the system.

Example Calculation Approach:
If the maximum output of the system is, for example, 90 liters, the hydraulic tank should ideally be sized so that it maintains a total capacity of at least one-third of this maximum output, at drawdown, which is 30 liters (90 liters/3)

This calculation ensures that even when the hydraulic system is at maximum demand, there will still be 30 liters of fluid in the reservoir, allowing for proper residence time and contaminant settling. This setup protects the system from running the pump dry and ensures the hydraulic fluid is properly conditioned before re-entry into the system.

The issue of thermal expansion in hydraulic systems is important, as it affects the entire system volume rather than just the volume inside the reservoir. This is particularly relevant when the hydraulic fluid heats up and expands during operation.

Key Factors in Thermal Expansion Calculation:

1. Min and Max Level Setting: Setting the correct minimum and maximum levels in the tank is critical. The differential between these levels should account for the expansion volume of the oil. The oil levels should allow for the expansion when the system heats up, ensuring that the reservoir does not overflow when the oil is hot.
2. Reservoir Management: During the heating phase, as oil expands, excess air is vented through the outlet valve of breather cap. When cooling, the air volume displaced by thermal expansion is drawn back into the tank through the inlet valve of breather cap. Since this happens slowly and progressively, the risk of contamination entering the tank is low, especially if the breather or filtration system is well-maintained.
3. Contamination Concern: Properly designed systems with adequate filtration and breather systems reduce the likelihood of contamination entering the tank during the expansion and contraction phases. If the system is properly sealed and the breather is functioning, the risk of contamination remains minimal.

To properly size the hydraulic tank, it is beneficial to consider below mentioned points.

3D modeling helps assess how the tank will perform under various conditions, such as gradients. When the machine is at an allowable angle, the oil inside the tank will shift accordingly. Through 3D modeling, you can determine whether the suction strainer remains submerged at both minimum and maximum oil levels, ensuring optimal performance.

CFD Simulation for hydraulic tank can be done in static and dynamic condition.

Static Simulation

This simulation helps determine if the minimum and maximum oil levels are accurately calculated based on the breather’s pressure settings. It also evaluates the distribution of return flow into the tank and checks if air is being drawn into the suction line or properly settling out within the tank.

Dynamic Simulation

Addition to the static simulation results, this test replicates real-world conditions, showing how oil behaves during machine acceleration, deceleration, and when the machine is at different angles. Based on these results, the tank design can be optimized, such as adjusting the baffle plate design or relocating the suction port for better performance.

In conclusion, designing and sizing a hydraulic tank is crucial for maintaining the efficiency and longevity of hydraulic systems. A properly designed tank not only stores hydraulic fluid but also helps condition it, preventing issues like cavitation and contamination. This article has covered the essentials of hydraulic tank design, including types, functions, and advanced considerations.

Both industrial and mobile hydraulic tanks have specific requirements, with mobile tanks needing custom designs to fit compact spaces and endure harsh conditions. Key design elements include ensuring adequate tank capacity, managing thermal expansion, and implementing effective pressurization and baffle designs.

Modern tools like 3D modeling and CFD simulations are essential for optimizing tank performance. By focusing on these principles, you can design a hydraulic tank that meets operational needs, maximizes efficiency, and minimizes costs.