The viscosity of a chemical cargo determines how easy it is to pump out the product and the amount of residue that will be left after discharge .
Viscosity is internal resistance of a fluid to flow or a measure of fluid friction---thus water is thin having low viscosity and honey is thick having high viscosity.
Viscosity is a measure of a liquid’s ability to flow and is usually determined by measuring the time required for a fixed volume to flow under gravity through a thin tube at a fixed temperature.
As the temperature of the liquid increases its viscosity decreases and therefore it flows more readily. It can also be described as a measure of the internal friction of a liquid.
The distinction between viscosity and pour point should be made clear. Oil ceases to flow below its pour point temperature when the wax content solidifies.
A viscosity measurement of a liquid depends upon the internal resistance of the liquid to flow. For a simple liquid this internal resistance varies with the temperature in a predictable and regular way.
However, when oil approaches its pour point the rate at which viscosity increases as temperature falls accelerates until sufficient wax has precipitated to solidify the product.
Viscosity is important as regards the pumpability of a product. Centrifugal and deepwell pumps are acceptable for the majority of cargoes but high-viscosity products such as bitumen or molasses are more suited for pumping with positive displacement pumps.
High-Viscosity Substance means a noxious liquid substance in Category X or Y with a viscosity equal to or greater than 50 mPa.s at the unloading temperature. Low-Viscosity Substance means a noxious liquid substance, which is not a High-Viscosity Substance.
MARPOL Annex II requirement for high viscosity substances are to be complied with.
There are time charterers who will put your ship off hire when you cant use your deepwell centrifugal pump to discharge high brix molasses or clay slurries--- viscous cargoes which need a screw pump.
After 1.1.07 with Cat Y/ Z Marpol categories pre-wash depends on viscosity of cargo at DISCHARGE PORT temperature --it becomes all the more imperative to know this branch of Physics—how increase in temperature reduces viscosity.
High viscosity chemical should be PRE-washed with hot water, with cold interface removed, and the washing discharged to shore reception facilities..
In a laymans language it is a measure of the ability of a liquid to flow through a hole. (For the highest common denominator-- The force per unit area needed to shear a fluid is proportional to the velocity gradient. This constant of proportionality is the viscosity.)
This resistance acts against the motion of any solid object through the fluid and also against motion of the fluid itself past stationary obstacles. Viscosity also acts internally on the fluid between slower and faster moving adjacent layers.
All fluids, exhibit viscosity to some degree. Viscosity may be thought of as fluid friction.
Viscosity diminishes as temperature rises, often by about 2% per degree C. Certain cargoes like like Lub Oil Additives show greater viscosity when heated.
The Poiseuille and the Poise are units of dynamic viscosity sometimes called absolute viscosity.
1 Poiseuille (Pl) = 10 poise (P) = 1000 cP
Phonetically it is spelled pwaz. Poise (P) named after Jean Louis Marie Poiseuille. It is more commonly expressed, particularly in ASTM standards, as centipoise (cP). The centipoise is commonly used because water has a viscosity of 1.0020 cP (at 20 °C) 1 poise = 100 centipoise = 1 g/(cm·s) = 0.1 Pa·s.
1 centipoise = 1 mPa·s.
Screw pumps can handle 1 million mPas or cP.
For the VCS system: While the viscosity of solids and liquids falls with increasing temperature (inverse relationship), the viscosity of a gas increases.
Another surprise is that pressure has very little effect on viscosity. Viscosity’s virtual independence from pressure means that water flowing in a pipe has an insignificant change in friction whether at 60 psi or 20,000 psi!
Water @ 99°C/ 0.2848 cP
Acetone/ 0.3 cP
Benzene/ 0.50 cP
Methyl alcohol@ 20°C / 0.597 cP
Water @ 20°C / 1.002 cP
Motor oil SAE 10 / 65 cP
Soya bean oil @ 20°C/ 69.3 cP
Corn oil/ 72 cP
Olive oil @ 20°C/ 84.0 cP
Motor oil SAE 20/ 125 cP
Motor oil SAE 30 / 200 cP
Motor oil SAE 40/ 319 cP
Motor oil SAE 50/ 540 cP
Caster oil @ 20°C / 986 cP
Motor oil SAE 60/ 1,000 cP
Ethyl alcohol @ 20°C/ 1.2 cP
Motor oil SAE 70/ 1,600 cP
Molasses @25°C / 8,700 cP
Honey @ 20°C/ 10,000 cP
Corn Syrup/ 110,000 cP
Tar or pitch @ 20°C/ 3x1010 cP
The "thicker" the fluid, the greater its resistance to shear stress and the more rapid the decay of its flow.
Kinematic viscosity: The SI physical unit of kinematic viscosity is the (m2/s). The CGS physical unit for kinematic viscosity is the stokes (abbreviated S or St), named after George Gabriel Stokes . It is sometimes expressed in terms of centistokes (cS or cSt).
1 stokes = 100 centistokes = 1 cm2/s = 0.0001 m2/s.
Viscosity can be a major factor that affects the value of the Reynolds number.
The Reynolds number is the ratio of inertial forces to viscous (drag) forces and is used for determining whether a flow will be laminar or turbulent.
Laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion, while turbulent flow, on the other hand, occurs at high Reynolds numbers and is dominated by inertial forces, producing random eddies, vortices and other flow fluctuations.
The transition between laminar and turbulent flow is often indicated by a critical Reynolds number. Within circular pipes the critical Reynolds number is generally accepted to be 2300
If we want to calculate the Reynolds number , we can use the following equation.
R = 3160 x Q x Gt D x μ
where: R = Reynolds number Q = liquid's flow rate, gpm Gt = liquid's specific gravity D = inside pipe diameter, in. μ = liquid's viscosity, cp
Critical velocity= Visc coefficient X RN/ SG X dia
When the Reynolds number is less than 2000, flow will be described as laminar When the Reynolds number is greater than 4000, flow will be described as turbulent When the Reynolds number is in the range of 2000 to 4000 the flow is considered transitional.
For e.g You may find highly viscous hydraulic oils may exhibit laminar flow in most conditions while things like water will be turbulent.
NOW LET US DIGRESS BACK TO CHEMISTRY
Products with a high viscosity should be washed at higher temperatures. In general the viscosity is closely related to the temperature and will decrease at higher temperatures. During washing there should be no ballast water or cold cargoes adjacent to the tank to be cleaned.
Washing as soon as possible after discharge is recommended. The respective product characteristic of the individual product must be observed.
Exceptions are some highly viscous cargoes (e.g. certain lube additives or polyols) which need to be pre-cleaned with lower or moderate temperatures, because they tend to undesired reactions at higher temperatures.
Optimum viscosity for pumping fuel oil is 400 Cst. Max viscosity limit for practical pumping purposes is 1250 Cst
Transformer oils are used in elec equipment as coolant and as insulating material and are very sensitive to alterations of viscosity.
Cargoes with high viscosity also require a higher temperature wash water. The wash water then has to be heated to bring the viscosity to the chemical to at least below 20 to 60 mPas (cp.).
Remember the HO bunkers taken by chief engineer is reduced in viscosity from 380 cst to 15 cst prior to injection ino the ME—the heating to be done, say upto 135 deg C is known to him.
Some high viscosity cargoes eg. Lub oil additives have to be precleaned with warm water before a hot wash.
Some chemicals like LO additives, veg/ animal oils, polyols etc do not have a defined MP, but a range. For such cargoes viscosity is used as a measurement of the products liquidity of handling characteristics, and the term pour point is used instead.
Fluids are broadly classified a Newtonian and non Newtonian depending upon their obedience to the laws of classical mechanics. For a Newtonian liquid rate of flow is proportional to the force applied. Some fluids will not flow till the force exerted is greater than the yield point.
Pumpability depends on brix value, its place of origin, storage time, fermentation process, and last but not least, temperature during period the cargo is on board the ship. BRIX indicates pumpability of product.
SG=400/400-brix in deg
Molasses exhibits the phenomenon called critical viscosity which means that above a certain dry matter content the viscosity increases at a greater rate than might be expected from the increased dry matter content. The critical viscosity for cane molasses lies between 81 and 85º hydrometer brix.
The viscosity of molasses is affected both by dry matter and temperature, for example a rise in temperature of 10 C may reduce the viscosity to half or less and a reduction in the dry matter content will also decrease viscosity.
Low Brix molasses are easily discharged by Framo deepwell pumps. However those with Brix number 89/93 are considered very thick like honey and impossible to discharge with a centrifugal pump. A screw portable framo pump has to be arranged , and a good booster pump ashore.
SCREW PUMPS are self priming. The rotating elements are axially balanced and there is no metal to metal contact within the pump between the screws or the body .
They can handle virtually any non homogeneous liquid regardless of viscosity and abrasiveness—molasses, caustic soda, sea water , bitumen, clay slurry etc. generally used on a chemical tanker as a booster pump for molasses.
It can handle a highly viscous liquid of up to 1,000,000 mPaS like treacle molasses of high Brix value , as well as a material that is almost non-fluidal. The liquid can be transferred smoothly without pulsation, stirring, or foaming.
Furthermore, the flow rate can be controlled easily by adjusting the pump speed. The self-priming capability is maximum 9 m, making the pump ideal for assisting the deepwell pump by sucking from underneath –from the cargo tanks Drive is usually Framo powered –they are able to withstand high temp and backpressures
Category Y, high Viscosity and solidifying cargoes may require prewash, if not heated.
High-Viscosity Substance means a noxious liquid substance in Category X or Y with a viscosity equal to or greater than 50 mPa.s at the unloading temperature.
Solidifying Substances and substances with viscosity equal to or greater than 50 mPa.s at 20°C shall be washed with hot water (temperature at least 60°C) when water is used as the washing medium, unless the properties of such substances make the washing less effective.
CAPT AJIT VADAKAYIL
30 YEARS IN COMMAND