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When connecting a DC hydraulic power pack to the battery it is important to get the cable sizing correct. The battery cable will carry high currents and they must be sized to minimise voltage drops. When a DC powerpack is running at full load currents up to 300 amps can be required.
Maximum current draw of your DC powerpack can be estimated as follows, but for exact current figures the dc motor performance curves must be referred to.
On many vehicles and trailers the battery can be mounted some distance from the power pack the length of the battery cables must be considered. This is because the voltage drop calculation is proportional to the resistance of the battery cables and therefore the amount of copper used.
These figures are based on the use of a Tri-rated cable (BS6231) as this a flexible and high temperature multi-stranded high current cable. Battery cable must be terminated with crimped ring ends, ensuring these are securely tightened onto battery and dc hydraulic power pack terminals.
Failure to adhere to these guidelines can cause excessive voltage drop in the cables at full load current. This can cause damage to the power pack and also result in dangerous overheating of the cables and connected equipment. The use of a suitably sized fuse is recommended.
There are a wide range of choices over an even wider range of budgets, but the right hydraulic oil will prolong your machine life and reduce your overall running costs.
Three initial questions must be answered:-
1) In what type of equipment will the hydraulic fluid be used?
2) How severe will the duty be?
3) What operating temperature and pressures will be experienced?
4) Environment food safe etc
Answers to these questions will lead to the primary choices of viscosity grade (VG) and hydraulic fluid types.
In what type of equipment will the hydraulic oils be used?
Selection of a hydraulic fluid with a viscosity that bests suits the system pump is a good place to start. Manufacturers will normally specify a range of oil viscosity. These will vary dependent upon the pump type. Vane pumps typically require 14-160 cSt, Piston pumps are more durable than a vane pump and require 10-160cSt. Gear pumps are the most tolerant to contamination and a conservative range would be 10-300cSt. Industrial machinery is typically designed to operate within a cleaner more stable environment, where outdoor and mobile applications will more likely have severe temperature variations, higher humidity and more demanding duty cycles.
How severe will the duty be?
Duty would normally be described by running time, environmental factors, likelihood of contamination ingress, maintenance arrangements etc.
Examples of Low/Medium/Heavy Duty would be:-
> 24 hours
Heavier duty demands will normally lead to the use of a mineral oil with a good additive package (such as a HVLP) to improve performance or the selection of a fully synthetic oil.
For hydraulic systems with high running times a fluid with a high viscosity index (VI>130) will avoid damage and breakdowns as it extends lifetime of hydraulic pumps and components.
What operating temperature and pressures will be experienced?
Where temperature extremes are large (below -5oC and above +60oC) and pressures above 250 bar the use of a fluid with a good mix of additives will be important. Mineral based oils (HM/HLP) will be sufficient in the most common applications as these often have anti-wear additives, oxidisation inhibitors and viscosity improvers. Fully synthetic oils will however out-perform mineral hydraulic oil ensuring that the viscosity and lubricity remains stable over a longer period.
Viscosity Grade (VG)
A hydraulic fluid has a low viscosity when it is thin and a high viscosity grade when it is thick. The viscosity reduces as the temperature rises and visa-versa. The hydraulic fluid must be thin enough to flow through the filter, inlet and return pipes without too much resistance. On the other hand, the hydraulic fluid must not be too thin, in order to avoid wear due to lack of lubrication and to keep internal leakage within limits. Viscosity grade is expressed at 40oC eg ISO46 which is an oil with a viscosity of 46 cSt measured at 40oC.
According to DINISO 2909 oil viscosity changes versus temperature, Viscosity Index (VI), is normally between 90-110. VI above 130 are largely insensitive to temperature change.
A viscosity range of 12-80sCt is recommended for a large range of commercially used hydraulic equipment.
Hydraulic oil specifications
Hydraulic power packs can be used with a wide range of hydraulic oil grades, commonly:-
· Hydraulic Oil (ISO11158-HM) – Mineral based – hydraulic oil grades widely used in light duty applications where temperature and pressures are moderate.
· Hydraulic Oil (DIN51524-2-HLP) – Mineral based with additives for oxidation, corrosion and wear protection. Used for general applications where temperature and viscosity conditions are observed.
· Hydraulic Oil (51524-3-HVLP) – Premium grade mineral based as per HLP but with improved viscosity temperature behaviour (VI>140).
· Biodegradable hydraulic oil – HETG, HEPG, HEES and HEPR – A developing technology and is yet to replace mineral oils in all applications. Storage and service life is limited, particularly at elevated temperatures.
· Fire Resistant Fluids (ISO12922 – HFA, HFB, HFC and HFD) – HFA,HFB and HFC contain water solutions and must only be used with specifically designed products. Not suitable for systems containing aluminium and some paint products. Seal compatibility must be checked.
For Hydraproducts powerpacks we recommend the following:-
HPU and HPR Micro powerpacks
HPM Mini packs
HPS Standard Hydraulic power units
Some sources of these oils would be:-
HM32 – Shell Hydrau HM32 – Castrol Hyspin VG32
HLP32 – Shell Tellus 32 – Castol Hyspin AWS32
HVLP32 – Shell Tellus S3V 32 – Castrol Hyspin HVI 32
Where environmentally sensitive fluids are required the use of Castrol Carelube HES32 can be employed in all our products, for light and medium duty ONLY.
Where a small level of fire resistance desirable then the use of a Castrol Anvol SWX FM HFDU fluid may be implemented in all of our products, for light and medium duty ONLY.
Is it just about “three times flow rate”?
The widespread use of the “three times flow” rule of thumb serves well but current pressures on space, economics and environmental issues warrant a closer examination of this rule.
So what are the factors to be considered?
• Hold enough oil for system function
• Sufficient surface area to dissipate heat to the surroundings
• Large enough volume so turbulence is minimized allowing entrained air to escape and contaminations to settle
• Separating the suction from the return areas.
• Access for maintenance and cleaning
• Air space conditions, pressure, dryness and cleanliness
• “Real-estate” for fitting of main system components
Basic features of a traditional oil reservoir:-
Offshore containers for Hydraulic Power Units (HPU)
Hydraulic equipment that is used and transported on and off ships is often housed within a container. Hydraulic power units are commonly required for Launch and Recovery Systems (LARS), Remotely Operated Vehicles (ROV) and Winches. This would be normally designed to meet Offshore or Maritime standards.
The offshore containers are designed for moving around supply ship decks and must withstand dynamic loads from “snatching” of installation cranes.
The portable offshore container has a maximum gross weight less than 25,000 kg for continuous transport of goods and equipment, and will be handled in open seas between fixed or floating platforms and decks. This differs from the containers used in the containerised transport of goods on a ship.
The offshore container will be designed and certified to meet the DNV Standard for Certification No 2.7-1 (April 2006) with effect from in 1989. This standard was introduced as the sea transport container standard (ISO 1496) was not intended offshore containers.
Basically the offshore container must be “tough” to support this DNV 2.7-1 focuses on strength and integrity in all stages of transport.
· shoreside (by fork lift truck)
· supply vessel
· craning on and off offshore platforms
Containers must have sufficient strength so not to suffer failure when subjected to extreme loads that may be encountered during offshore movements.
Containers designed according to DNV 2.7-2 will largely meet the design and certification needs to also satisfy CSC (Convention for Safe Container), IMDG (International Maritime Dangerous Goods Code) and the ISO1496 standards.
Whilst DNV 2.7-2 focus generally on the dimensions and weights another standard that runs alongside this is the European Standard EN12079. The bulk of this standard concentrates on the safety and recording documentation. Sections of EN12079 (Offshore containers and associated lifting sets) will shape the general design:-
● An offshore container should be designed to allow loading and unloading in wave heights to 6 m
● Must have an outer framework and pad eyes at or near the top
● Protrusions to be avoided
● Designed to withstand temperature of -20℃
● Must withstand 30˚ angle without tipping
The owners or representative are responsible for recording and maintaining certification of all containers. Periodic inspection including records of substantial repair, modification must ensure traceability.
Transit crash frames give essential protection for Offshore HPU’s. Hydraproducts can offer guidance on how to comply with the standards necessary when handling marine quality hydraulic power packs.
The inspection, maintenance and survey support asks that the ROV performs include:
· Inspection, maintenance and survey support activities
· Marine growth survey
· Flood member detection survey
· Cathodic protection survey and calibration
· Close visual inspection
· General visual inspection
· Scour and debris survey
· Pipeline inspection / survey
· Offshore structure cleaning
Other tasks include these drill related activities:
· Bullseye reading
· Water jetting / dredging
· Seabed samples
· Seabed survey
· Drill string assistance
· Gas bubble watch
If you’re looking for help with your subsea hydraulic powered machinery, contact us today for a no obligation chat and we may be able to help you to solve your marine related industry challenge.
A design decision to fit a hydraulic oil cooler like this can only be made with a full assessment of how much heat is going to be put into the oil and how much of that heat can the system dissipate without the oil overheating.
The heat comes from inefficiencies or losses. The greater the inefficiencies the greater more power input will be required to overcome them. The inefficiencies will manifest themselves as heat. So the amount of extra input power required can be assessed as:-
Input Power due to inefficiencies = Power Loss (pump) + Power Loss (valves) + Power loss (pipework)+….
This is often described as “heat Load” and if this is greater than the system heat dissipation the system will overheat without a hydraulic cooler.
Where does the heat come from?
The most common cause of heat is pressure drop, this being a decrease in pressure across a device as the oil flows through it.
Sizing of control valves and pipework is critical to reducing heat sources, for example a Cetop03 Directional Valve that is rated at 80lpm with 50lpm of flow from the A to T ports may have a pressure drop of 10 bar would generate (10*50/550) 0.9kW of heat.
Selection of high efficiency pumps and motors is also important, for example a Gear pump that is driven by a 11kW motor would typically have an efficiency between 90 and 95%. This will mean that the output power of the pump will only be 10 to 10.5kW, so 0.5 to 1kW will be heat.
From the initial design of the whole system it is possible to make an estimation of the total heat load. The heat load of an existing system can simply be estimated by measuring the oil temperature rise over time.
Will my powerpack dissipate enough heat without a hydraulic cooler?
Typically the oil reservoir will be sized with heat dissipation in mind. Lets not forget that a system that operates for 1 minute every hour will put a lot less heat into the oil and has much longer to dissipate that heat.
Tank heat loss can be expressed as :- kW= DT * A * 0.016
DT = temperatute difference between oil and ambient (oC)
A = Surface area of tank, generally excluding base (m2)
For example a 55Litre tank could have a surface are of 1m2 and if there is an ambient temperature of 20oC, and the oil temperature is 60oC, then the tank will dissipate 0.7kW.
The maximum oil temperature allowable will depend a little on the type of components and oil used, but in general temperatures above 80oC will damage most seals and degrade performance of many commonly used hydraulic oils.
All of the system will dissipate heat, pipes, fittings and valve bodies etc. So for a full system heat loss figure the total surface area of the system could be considered. In systems where pipework and hydraulic cylinders are large this surface area could be worth consideration.
If the system heat loss is less than the heat load then, yes the hydraulic system will require hydraulic oil coolers if it is to be 100% duty rated.
Keep cool and improve the design
Sources of heat can be designed out of systems by careful selection of the correct valves and by setting them appropriately.
Some of the simple design errors to avoid:-
• Ensure the pump is not operating at full pressure while it is not in use. This will cause the system relief valve to by-pass to tank at full pressure drop. This will cause a heat input equal to the full input power. It is always recommended to use an loading valve so the pump by-passes to tank at the lowest possible pressure drop.
• Slowing cylinder or motors down using a throttle or flow control valve. If a system has a fixed displacement pump then the use of a flow restriction like this cause the pump pressure to increase to the pressure setting of the system relief valve. So the cylinder or motor is only slowed because some of the pump output is by-passing to the tank via the pressure relief valve. Correct pump selection is vital if this situation occurs.
• Load control valves such as overcentre or counterbalance valves that are set too high will cause the system to require excessive pressure when lowering. The pump will have to work harder than necessary.
• Pipework sizing, can be misunderstood, particularly when long hoses are used. For example a 30metre long 3/8” hose with 50lpm flow will have a 15bar pressure drop.
If your system is to run continuously then firstly ensure that system design is optimised and then consider the heat load. A mixture of system heat dissipation and the use of hydraulic coolers will always ensure your powerpack doesn’t overheat.
Hydraulic Power Pack
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