Viscosity measurement

INTRODUCTION

For efficient and complete combustion, residual fuel must be heated before it is burnt. For correct atomisation in the cylinder, the fuel must be at the correct viscosity. If the viscosity is too high, the fuel droplets will tend to be too large and will take too long to absorb the heat energy from the compressed air before they start to burn. This will lead to late and incomplete combustion, lack of power, afterburning and damage or fouling to liner, piston crown, exhaust valve and turbocharger. If the viscosity is too low, then the droplets will be too small and combustion will tend to be early and incomplete because the fuel droplets will not have penetrated far enough into the cylinder to find sufficient oxygen to burn completely. This again will cause damage and fouling.

Because residual fuel is a complex blend of heavy asphaltenes blended with lighter distillates, and will vary in its make up, to ensure that the fuel is maintained at the correct viscosity for injection, an inline device is used to measure the viscosity and maintain it at the correct setpoint by controlling the fuel heating source.

Marine Engineers often refer to this device as a viscotherm, although this is a trade name for a particular make of viscosity controller.

The Mechanically Driven Capilliary Tube Viscotherm

This is manufactured by VAF Instruments (Netherlands) Ltd.

1. Thermometer 2. damping capilliary 3. measuring capilliary 4. housing 5. d.p. transmitter connections 6. motor 7. reduction gear 8. magnetic coupling 9. gear pump

The sensor consists of a housing (4) in which the measuring element, a capillary tube assembly (2,3) is mounted together with a gear pump (9). An electric motor (6) with reduction gear (7) drives the pump such that a continuous and constant flow through the capillary tubes is achieved. The laminar flow through the measuring capillary creates a pressure differential which is proportional to the dynamic viscosity of the fuel oil. A magnetic coupling (8) prevents both leakage and overload of the electric motor in the event of an obstructed pump. The capillary assembly consists of a measuring capillary in a resilient stainless steel housing (3) and a stainless steel damping capillary (2) which compensates for pressure waves in the fuel lines. Pressure taps (5) are provided to connect the inlet and outlet of the measuring capillary to a differential pressure transmitter via a valve manifold. The manifold is used to isolate the differential pressure transmitter connections and to equalize the differential pressure across the transmitter. An optional thermometer (1) indicates the actual fuel temperature.

Exam type drawing explaining how a viscotherm works

The drawing above shows a pneumatic control system for controlling fuel oil viscosity. Modern systems convert the differential pressure to an low voltage electrical signal between 4 - 20 mA DC and use this to control the position of the heater valve.

Electronic Viscosity Measurement.

Systems are manufactured by VAF (Viscosense), Alpha Laval (Viscochief) and Solartron Mobrey (Viscomaster). Basically the systems all use the principle that the damping of a vibration signal is proportional to the square root of the viscosity. Not affected by vibrations or pressure and flow fluctuations. No moving parts to wear out.

THE VISCOSENSE

The sensor comprises of a stainless steel pendulum attached to a base plate via a torsion tube. two piezo elements are driven by an alternating signal causing the pendulum to vibrate. A second set of elements sense the torsional vibration via a feedback, and a processor measures the phase difference between the transmitted and received signals. This phase difference is processed which results in a value proportional to the square root of the fuel oil's viscosity.

The Viscochief

Fuel oil viscosity is measured by the damping effect of the oil on an oscillating rod.

Working Principle A pin (1) is always kept at its resonance frequency by a motor coil (2). A pick up coil (3) measures the amplitude change due to viscosity change. A protection tube (4) with 3 slots encloses and protects the vibrating pin and limits the flow. The electronic unit (5) consists of two circuit boards connected to each other by a flat cable. Measuring the decrease of the signal from the pick up coil compared to the input by the motor coil gives a value related to the viscosity.

The Viscomaster

The Viscomaster uses a vibrating tuning fork where the tines are electronically maintained in resonance. Unlike similar sensors, the instrument also displaces the fluid, and this allows it to measure density and viscosity concurrently. Resonant frequency is determined by the density of the fluid, and the damping of the vibration is proportional to its viscosity. The damped vibration is measured by the pick up piezo resistive elements and a signal sent back to the processor which measures the difference between the drive and pick up and produces an output signal between 4 and 20 mA which is used to control the F.O. heater steam inlet valve.