Is Stainless Steel SS316 resist to CAVITATION ?

RECALL...
As everybody aware that Stainless Steel (i.e. SS316) is "soft" compare to other material e.g Cast Iron, Duplex stainless steel, etc.

Why is stainless steel so good against cavitation ?
Wouldn't the imploding bubbles erode Stainless steel which is rather "soft" ?



This main due to work-hardening property of Stainless Steel. As one hammering a SS316 strip, you may notice that the surface work-hardens and difficult to change the shape.

Similar phenomenon occurs when fluid bubbles imploding and impacting on the surface of stainless steel. Implosion of bubble causing the Stainless steel work-hardens, and increase resistance to further cavitation.

As per expert advice (Pump Magazine), SS316 resists cavitation about 10-15 times better than cast iron whilst CA6NM (modified SS316) is roughly 2-3 times more resistant to cavitation as compared to SS 316.

Amazing !!!

JoeWong

Pump Cavitation Phenomenon & How to avoid

Cavitation is the formation and collapse of vapor bubbles in a liquid.

Following are some examples of impeller damage by cavitation.

Bubble formation occurs at a point where the fluid operating pressure is lower than fluid vapor pressure, and bubble collapse or implosion occurs at a point where the pressure is increased to the vapor pressure. In general, cavitation occur at pump suction with lowest possible operating pressure. Figure P1 below shows a typical pressure profile in a centrifugal pump. As pumping fluid passing pump, operating pressure drop due to frictional lose (Entrance loss (path A-C). Once the liquid enters pump chamber, it will experience serious turbulence cause by impeller. Major Turbulence Friction Entrance Loss is expected along path C-D. Once the fluid reach point D, impeller generated large centrifugal force and acting on the liquid. The energy is transferred from pump impeller to liquid and increase liquid velocity and operating pressure. As the liquid is leaving the pump exit chamber, fluid velocity is reduced (expansion) velocity head is converted to pressure head base on Bernoulli principle (by Daniel Bernoulli) . This will further increase the fluid operating pressure (path D-E)


Figure P2 below shows as fluid B with low vapor pressure below lowest operating pressure in pump, NO cavitation occur. However, fluid A with high vapor pressure, as the operating pressure lower than fluid vapor pressure bubble form. Once fluid passing the impeller, operating pressure increased will cause bubble collapse (sometime called implosion) once the operating pressure above the vapor pressure. Above phenomenon occur in a very short time and it cause several things happen at once : · Bubbles collapsed when they pass into the higher regions of pressure, causing noise and vibration· Loss in capacity. · No longer build the same head (pressure) · Efficiency drops· Damage to many of the components i.e. chamber, impeller, etc.

One shall understand that pump chamber and impeller design will serious affect the entrance friction loss and turbulence loss caused by impeller. Refer to figure P3 below. Pump A having high entrance friction loss and turbulence loss results cavitation occurred. However, pump B shows low entrance friction loss and turbulence loss, operating pressure is always above vapor pressure and NO cavitation will occur.

Thus Process engineer must always ensure the operating pressure along the pump always higher than fluid vapor pressure. Generally Net positive suction head (NPSH) is used to check if cavitation will occur. Process engineer must always ensure available Net positive suction head (NPSHa )is always higher than pump required Net positive suction head (NPSHr).

Golden Rule ==>

NPSHa > NPSHr

The following chart illustrate the relationship between NPSHa & HPSHr

As NPSHr is subject physical construction of pump (by manufacturer), it is not much a Process Engineer can do other than specifying the requirement and selection of correct pump. However, Process engineer can put extra effort to increase NPSHa.

There are few ways to increase NPSHa :

a) Increase suction line size to reduce friction head loss. Generally a flow velocity less than 1 m/s

b) Rearrange and /or redesign suction pipe work to minimise bends, valves and fittings

c) Raise suction vessel

d) Increase & maintain pressure in suction vessel

e) Reduce fluid vapor pressure i.e. subcool fluid

From process perspective, step (a) to (c) are common apply to raise NPSHa as they can be implemented easily. As for step (d) & (e), they involve new equipment & control devices and directly increase CAPEX and OPEX of a project. Generally not advisable to apply unless all efforts are implemented.

(There are other factors & phenomenons causing pump cavitation e.g. gas entrainment, recirculation, etc...will discuss next day...to be continued)

JoeWong