To work out how much oil a hydraulic reservoir can hold, you need to calculate the flow rate of the pump, how much heat is produced by the system, and how long the oil spends in the reservoir. This article will explain the most important things that affect how much water can be stored, the most common ways to work out the size, and give you some example calculations. This will help you to work out the right size reservoir.
Why Does Hydraulic Reservoir Sizing Matter?
The amount of water in the reservoir affects how well the system cools down, how well bubbles are expelled, and how well contaminants settle. The size of the hydraulic reservoir is more important than the tank’s external dimensions for system stability.
Cooling and Heat Dissipation
While it is operational, the system constantly generates heat, which the hydraulic reservoir has to get rid of. If the reservoir is the right size, it makes it easier to control the oil temperature.
If the capacity is too small, the oil temperature rises faster. High temperatures can damage the oil and seals.
Oil Residence Time and Deaeration
After you return the oil to the reservoir, it needs to spend enough time there so that air can escape. The best time to wait is between 30 and 60 seconds.
If the reservoir is too small, oil flows too quickly, and air can’t escape. This can stop the pump from working properly.
Contamination Settling and Pump Stability
The reservoir also allows unwanted substances to settle naturally at low flow rates. If there is enough capacity, the flow of liquid through the system slows down, which helps to settle any contaminants that are in the liquid.
If there is not enough capacity, the flow is turbulent, and the contaminant does not settle. If you keep these conditions going for a long time, it can wear out the pumps and valves.
A System-Level Decision
Sizing hydraulic reservoirs is a system-level design issue. When working out how much a pump can handle, you should think about how fast it can pump, how much heat it makes, and the conditions it’s used in. Don’t just use a multiplier to work this out.
What Parameters Determine Hydraulic Reservoir Size?
The most important things that affect how much water a hydraulic tank can hold are the speed of the pump, the speed of the return line, how often the system is used, and the maximum temperature increase. When working out how much water a tank can hold, you should calculate each number on its own instead of just using a multiplier based on experience.
Primary Variable: Pump Flow Rate
The most important thing to know is the pump flow rate. The tank needs to be able to hold the same amount of oil as the pump puts out each minute.
It is common to first guess that the volume is between 2 and 5 times the pump flow rate, but this is just a first estimate. The actual capacity needs to be adjusted based on how it is used.
Secondary Variables: Duty Cycle and Thermal Load
The system duty cycle shows the percentage of time the equipment is switched on. The longer the continuous runtime, the higher the thermal load, and the more heat the system generates.
To control the oil temperature, make sure it doesn’t go above 10–15°C. If you want to generate more heat, you need a bigger tank.
Return Flow and Oil Residence Time
The speed at which oil circulates the tank depends on the return line flow rate. If the flow rate is too high, the oil doesn’t have enough time to go through the pipes.
It is recommended that the oil be left in the reservoir for between 30 and 60 seconds. This helps air to flow through and makes it easier for sediment to settle, while also making the system more stable.
Practical Parameter Check
When it comes to practical design, follow this sequence: first, work out the pump flow rate, then check the duty cycle and thermal load, and finally make sure that the oil residence time meets requirements.
If you calculate using this procedure, you can make sure that the size of the hydraulic reservoir matches the actual operating conditions. This will make it more reliable and help you to choose the right capacity.
How to Calculate Hydraulic Reservoir Size?
The most common way to size a hydraulic reservoir is to first guess an initial capacity of 2 to 5 times the pump flow rate, and then adjust it based on how it actually performs when it’s being used. You should treat empirical values as a reference, not the final result.
Rule of Thumb: Oil Volume to Pump Ratio
When you’re working out the size of a hydraulic tank for a pump, it’s common to use a ratio of 2×–5× oil volume to pump volume.
For example, if you use a pump that can flow 60 litres of water per minute, you can calculate that it can hold around 180 litres. This easy method is good for doing a quick check.
If the system is used for a long time or has average heat conditions, a ratio higher than 3x is usually needed. If you only use it sometimes and have an oil cooler, a ratio of slightly less than 3× might be good enough.
Basic Hydraulic Reservoir Sizing Formula
There is a more straightforward way to work out the size of the hydraulic reservoir. You can use this formula:
Reservoir Volume = Pump Flow x Residence Time
This formula, based on residence time, helps to control flow conditions.
When to Increase or Reduce Capacity
Increase the amount of water in the reservoir when the system duty cycle is high, the thermal load is significant, or the temperature must be controlled within 10–15°C.
If the system is not too busy, the machine does not need to work for a long time, and the temperature is good, the machine can be made to work for less time without affecting how long it takes to get the job done.
By working through the steps and making changes, you can get better results for how big you think the hydraulic reservoir is. This is better than just using a fixed multiplier.
Example Calculation for a Real Hydraulic System
For a system with a 60 L/min pump working at 80% of its maximum capacity, the reservoir should have a capacity of between 30 and 180 L. The exact value depends on how much heat is generated and the cooling conditions.
Here is an example of how to calculate the size of a hydraulic tank:
Pump flow：60 L/min
Duty cycle：80%
Ambient temperature：35℃
Step 1: Residence Time Method
First, calculate the base capacity based on how long the water is in the tank. If you want the oil to stay in the tank for 30 seconds (0.5 minutes), follow these steps:
The volume of the reservoir is 30 litres.
The minimum capacity is 30 L, which means that only the basic oil is used.
Step 2: Temperature Rise Check
If you run it at 80% of its maximum capacity, it will produce heat all the time. At an ambient temperature of 35°C, the heat is not being dissipated as well as it could.
If you need to control the allowable temperature rise within 10–15°C, 30 L alone is typically not enough.
To meet the cooling capacity requirements, it is often necessary to increase the capacity to 120–180 L.
Small Tank vs Large Tank
A 30 L reservoir is small and compact, but it doesn’t dissipate heat very well, so the temperature is more likely to rise.
A 150-litre reservoir helps to control the temperature and keep the system running smoothly, but it takes up more space.
When it comes to practical design, you should choose a reasonable capacity range based on heat levels and how much space you have to install it in.
What Happens If a Hydraulic Reservoir Is Undersized?
If there is not enough oil in the hydraulic reservoir, the oil temperature will rise more quickly, it will be harder to get rid of air, and the pump will be more likely to get damaged. If you use it for a long time, it will affect how stable the system is and how long the parts will last.
Temperature Rise and Overheating
If the reservoir is too small, the oil has to circulate more often, but there is not enough space for the heat to get out, which can easily make the hydraulic tank overheat.
If the oil is too hot for too long, it will start to break down. You may see that the oil is darker and has a stronger smell.
Common symptoms of a hydraulic reservoir that is too small include frequent temperature alarms and slowed system response.
Pump Cavitation Risk
If the water doesn’t spend enough time in the reservoir, air can’t be fully released.
If air enters the pump suction port, it can cause the pump to malfunction. You might hear strange noises or see the pressure changing.
Prolonged cavitation can damage the internal parts of the pump.
Poor Contamination Settling
If there is not enough capacity, the flow speed inside is too high, which stops the dirt from settling.
Contaminants that have not yet been cleaned are constantly moving through the system, which damages the valves and pumps.
If you notice that the filter needs to be replaced more often or that there is more contamination, it may be because the reservoir is not big enough.
Can a Hydraulic Reservoir Be Too Large?
An oversized oil tank can lower oil temperature, but it costs more, takes up more space, and can affect how well the system works. The capacity design should be kept to a sensible level.
Cost and Space Impact
It costs more to make larger tanks because more material is needed.
On mobile equipment or devices with limited space, a tank that is too big can make it hard to install and can add weight.
When you’re working out the right size for a hydraulic tank, you need to think about how well it works and how much space it will take up.
Oil Circulation Efficiency
A tank that is too big makes it take longer for the oil to be replaced, which means the fluid in the system is renewed more slowly.
In systems that need to move quickly, having a lower flow speed can affect how quickly the system responds.
Keep the Capacity Within a Practical Range
Having more capacity isn’t always better. If you make it too small, there’s a risk of overheating and damage, while if you make it too big, it’ll cost more and add to the structural weight.
When working out how much it can take, choose a range that is stable, cheap, and easy to install based on the actual flow rates and heat levels.
Design Trade-Offs and Optimization Strategies
If you don’t have enough space or it’s expensive to install more tanks, you can still increase cooling capacity or make the return oil flow paths more efficient. Even if the tank size is fixed, the design makes sure the system works smoothly.
Use an External Heat Exchanger
If you need to cool a lot of water but there is not more space in the tank, adding a heat exchanger is a good idea.
An external cooler directly lowers the oil temperature, improving hydraulic oil temperature control. This means you can still control the temperature even if you have a smaller tank.
This is a common approach in compact hydraulic power units or equipment where space is limited.
Improve Heat Dissipation
If you can’t install a cooler, you can make the tank itself more effective at dissipating heat.
For example, make the outside of the tank bigger so that heat can escape more easily, and make sure there is good air circulation. This can reduce the oil temperature a little.
Optimize Return Flow Layout
If you arrange the return oil lines in the right way, you can make the oil in the tank flow more easily.
By improving the baffle structures to move the return port away from the suction port, bubble release and flow stability are made better.
Practical Choice Under Constraints
In mobile equipment or compact systems, the amount of fuel that can be stored is often limited.
In these cases, it is very important to include coolers, heat dissipation design, and return flow layout. This is because the space available is limited. It’s better to have a balanced capacity and cooling capability than to simply enlarge the tank.
FAQ
What is the minimum hydraulic reservoir size?
The size of the minimum hydraulic reservoir is usually calculated based on the pump flow and how long the oil spends in it. The formula for this is usually ‘Pump Flow × Oil Residence Time’, with residence time set at 30 to 60 seconds. This result only shows the basic capacity, which should be increased based on how much heat is being used.
Is the 3 times pump flow rule always correct?
The 3 times pump flow rule is a guideline based on experience, not a fixed standard. In systems with light loads or equipped with coolers, values below 3× may be enough. However, under high-load or high-temperature conditions, 4–5× may be needed. Determination should be based on the actual operating conditions.
How long should hydraulic oil stay in the reservoir?
It is recommended that the oil be left for between 30 and 60 seconds. This helps air to circulate and removes any dirt or other unwanted substances. If the pump doesn’t spend enough time in one place, it won’t be able to suck up water properly.
Does ambient temperature affect reservoir sizing?
The temperature affects how big hydraulic reservoirs need to be. Higher temperatures make it harder to keep the body cool. It is usually best to keep the temperature rise to between 10 and 15°C.
Can I reduce the tank size if I use a cooler?
When the cooling capacity is enough, the reservoir capacity can be reduced. If the heat exchanger can do the job, you don’t need to use a big reservoir. But the basic residence time must still be ensured.
How do I know if my reservoir is undersized?
If the oil gets hotter quickly and the pump makes more noise, it usually means that the reservoir is too small. Common signs that a hydraulic reservoir is too small include frequent temperature alarms, pressure changes, and faster oil wear and tear.
Is reservoir sizing different for mobile hydraulics?
The space available for equipment is smaller when it is for mobile equipment. Mobile hydraulics often use smaller tanks, which means they need to be cooled and laid out in a certain way. The calculation methods stay the same, but the adjustments needed might be more important.
What is the recommended temperature rise limit?
It is recommended to control the allowable temperature rise within 10–15°C. If you go beyond this range, the oil will start to break down more quickly, and it will be harder to keep the hydraulic oil at the right temperature. If we want to control the temperature rise more strictly, we need more power.
Final Thoughts
The right size for a hydraulic reservoir is worked out based on the pump flow rate, how much heat is produced in the system, and how long the oil spends in the reservoir. This is better than just using multipliers that are based on experience.
Target Hydraulics has over ten years of experience in choosing hydraulic systems and reservoirs. We can calculate how much water a reservoir can hold, analyse how to control the temperature of water, and suggest ways to improve your system. If you need more precise capacity solutions, please contact us.
