Question

Q1
a) With the aid of a sketch of the hydraulic system explain the following
i) A Reservoir ii) A pump and a prime mover iii) A pressure control and check valve iv) A power circuit which is made of actuators, motors or cylinders. v) Filter
b) List three functions that are performed by a reservoir and the purpose of baffles in it.
Q2
a) Define the term ‘Filtration’ in a hydraulic system.
b) Explain the difference between a depth and surface type filter and list three kinds of filters with different applications in a hydraulic system.
c) Name the three ratings used to classify hydraulic filters.
d) Explain what is meant by the terms;
i) Viscosity index
ii) Viscosity
iii) Demulsification
Q3
a) What is the procedure for isolating hydraulic machine before removing a component for servicing?
b) State a recommended procedure to use when you are testing a hydraulic line for leaks.
c) List two things you must take into consideration when you are using a cleaning solvent on hydraulic components.
d) State five contaminants that may be found in hydraulic systems.
Doc. No.: EXM/F/03 Page No.: 1/1 Issue No.: 1 Last Update: 15/04/08 Revision No.: 1 Date: 12/04/06 Author: EC Approved: DAA


Q4
a) Explain the term ‘Accumulator’ in a hydraulic system and its operation.
b) List the three different types of Accumulators used in a hydraulic system and explain the purpose of the nitrogen contained in it.
c) Draw the symbols of the following in a hydraulic circuit.
i) Accumulator
ii) Hydraulic pump
iii) Hydraulic motor
vi) Cylinder double acting
v) Ball check valve
Q5
a. State four different types of positive displacement pumps used in modern hydraulic systems and state the criteria on which a pump is selected for a system.
b. With a simple sketch describe the principle of operation of a gear pump.

Answer 1

Q 1

a]

1] A reservoir - The purpose of the hydraulic reservoir is to hold a volume of fluid, transfer heat from the system, allow solid contaminants to settle and facilitate the release of air and moisture from the fluid

2] A pump and a prime mover - A hydraulic pump is a mechanical device that converts mechanical power into hydraulic energy. It generates flow with enough power to overcome pressure induced by the load.

In a hydraulic system, the engine or electric motor that drives the pump, called the prime mover, is the source of power.

3]A power circuit which is made of actuators, motors or cylinders - In a basic hydraulic circuit, the force exerted by a cylinder is dependent upon the cylinder bore size and the pump pressure.This system is used where the cylinder piston is returned by mechanical force. With the control valve in neutral, pump flow passes through the valve and back to the reservoir.

4] Filter - A hydraulic filter helps to remove these particles and clean the oil on a continuous basis. The performance for every hydraulic filter is measured by its contamination removal efficiency, i.e. high dirt-holding capacities.

b]

Three Functions of Reservoir -

1] The main function of a reservoir is to hold system hydraulic fluid in a convenient location for the pump inlet.

2] In addition to system requirements, the reservoir also holds excess fluid needed when the hydraulic system is in operation.

3] It is used for heat dissipation (cooling the hydraulic fluid) and fluid conditioning (dissipation of contaminants and aeration).

Purpose of Baffes -  An internal baffle that is used to circulate turbulent fluid that is hot, dirty and aerated from the system return side of the reservoir to the quiet and cooler pump inlet side. This movement of the fluid around and through the baffle creates time for fluid contaminants to settle out to the bottom of the tank and for air entrapped in the fluid to separate and rise to the fluid surface.

Q 2

a] A hydraulic filtration system includes hydraulic filters to remove dirt and particles on a continuous basis.

b] Difference - Surface Filtration implies that particles are retained mostly on the surface of the media, forming a layer of material that increases the efficiency or fineness of particles retained. Depth Filtration refers to a thicker media or multiple layers of media, forming a torturous path to retain particles.

Types of Fliters -

1] In-line -  Pressure side – high pressure fluid & Suction side – lower pressure

  2] Off-line - Systems without continuous flow; systems where return flow volume is too great; lower pressure fluid.

3] Return line - Systems where pumps need the most protection; economic; casting machines, drilling rigs.

c] Flow Rate,Clogging Indicator, and Viscosity

d]

1] Viscosity index - The viscosity index (VI) is an arbitrary, unit-less measure of a fluid's change in viscosity relative to temperature change. It is mostly used to characterize the viscosity-temperature behavior of lubricating oils.

2] Viscosity - Viscosity is a measure of a fluid's resistance to flow.

3] Demulsification -Demulsification is the breaking of a crude oil emulsion into oil and water phases.

Q 3

a] The procedure for isolating hydraulic machine -

Preparation: Whether preparing a formal procedural document that could be incorporated in the Permit to Work (PtW), or simply getting ready to shut down the machine, preparation should include the following: 1. Risk assessment (i.e. job safety analysis, isolation method, isolation list). 2. Notification (inform anyone affected by the planned shutdown of the machinery). 3. Identify and gather any equipment needed to perform the lockout.

Shutdown: Switch off the machine following normal isolating procedure. In the case of hydraulic systems this is likely to include mains isolation switch, hydraulic supply valve, drain valve. Dissipate any stored energy via drain valves. Hydraulic supply valves can be locked in the closed position to prevent oil entering the system. Drain valves should be locked in the open position whereby hydraulic oil drains to the oil reservoir.

Lockout: Apply locking devices to electrical isolators and hydraulic system drain valves. Secure these in place with safety isolation padlocks. Padlock keys should be retained by the person who applied them or retained in a Group Lock Box.

Tagout: Apply signs or tags which provide the following information as a minimum. 1. Name of person carrying out the work. 2. Nature of work being undertaken. 3. Date tag applied. Signs and tags should be compliant with the signs and signals standards relevant to your location of work.

Blockout: As a final precaution, measures should be taken to prevent hydraulically powered components moving under their own weight. Locking pins may be provided as part of the machinery design. Where possible, these should be locked in place. In absence of this, blocks and chocks can be placed strategically to prevent parts from moving.

Restoration of system: Restoring the equipment or machinery to its normal operating condition is usually the reverse of the isolation procedure. Reference to the equipment or machinery hand books will advise on the correct sequence.

b]

When pinhole leaks develop in hydraulic lines, take the following steps to identify the locations of leaks and avoid injury:

• Before looking for the leak, shut off the power source to the hydraulic pump, and lower the implement to the ground.
• Relieve the pressure of the hydraulic system by moving the hydraulic control lever back and forth.
• To locate the leak, pass a piece of cardboard or mirror over the area where you suspect the leak.
• Fix the leak immediately.

c]

• Components that can be damaged by high fluid velocity or by fluids containing moisture, particles or flushing chemicals should be isolated from the flushing circuit and cleaned individually.

• Components that restrict the flow rate, and thereby increase the pressure drop, should be isolated from the flushing circuit and cleaned individually.

• Manifolds, blocks, pump stations, motors, reservoirs, assemblies and components should be delivered clean according to a specific procedure. If not clean, they must be flushed separately. This also applies where space does not allow flushing of installed piping system.

d]

• Metals.
• Silica.
• Elastomers.
• Fibers.
• Rust

Q 4

a] Accumulator - Hydraulic accumulators are pressure vessels that store and discharge energy in the form of pressurized fluid.

Operation -

In operation, the hydraulic pump raises system pressure and forces fluid to enter the accumulator. (Valves control oil flow in and out.) The piston or bladder moves and compresses the gas volume because fluid pressure exceeds the precharge pressure. This is the source of stored energy.

Movement stops when system and gas pressures are balanced. When a downstream action such as actuator movement creates system demand, hydraulic system pressure falls and the accumulator releases the stored, pressurized fluid to the circuit. When movement stops, the charging cycle begins again.

b]

Three different types of Accumulators - bladder, piston and diaphragm hydraulic accumulators

Purpose of the nitrogen contained - It is used as a compressible gas in accumulator.

c]

Q 5

a]

Four different types of positive displacement pumps - internal gear, external gear, timed lobe, and vane.

Criteria -

• Liquid viscosity.
• Temperature.
• Specific gravity.
• Vapor pressure.
• Solids present & concentration.
• Shear sensitive.
• Abrasive or Non-abrasive.

b]

Gear Pump Sketch -

Principle of Operation -

Gear pumps use the actions of rotating cogs or gears to transfer fluids. The rotating element develops a liquid seal with the pump casing and creates suction at the pump inlet. Fluid, drawn into the pump, is enclosed within the cavities of its rotating gears and transferred to the discharge.