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

Good documents on CRA & Corrosion control

Some good documents on CRA & Corrosion control found today...particularlly useful for those involve in Oil & Gas (O&G) and Exploration & Production (E&P) ...


“Performance of Duplex Stainless Steels in Hydrogen Sulphide Containing Environments”, Paper 207, Duplex Stainless Steels 1997, Maastricht, 21-23 October 1997. SMITH L.M. and FOWLER C.

“Control of Corrosion in Oil and Gas Production Tubing,” British Corrosion Journal, Volume 34, Number 4, 1999, P247 (Bengough Award for a paper with a strong industry overview given for this paper in year 2000). SMITH L.M.

“A Guideline To The Successful Use Of Duplex Stainless Steels For Flowlines”, Plenary Lecture, Duplex 2000, Houston TX, USA, 29 Feb - 1 March 2000. SMITH L.M, CELANT M. and POURBAIX A.

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

Unified Numbering System for Metals and Alloys

What are the differences between Duplex Stainless Steel, Medium Alloy Duplex, 22% Cr, SAF 2205 and UNS 31803 ?

Infact, all refer to same metal. Duplex Stainless Steel and Medium Alloy Duplex is general (layman) term and commonly used across discipline. Material specialist like to call it 22% Cr. SAF 2205 is the trade name where procurement people like put it in purchase order. Different terms used sometime may results confusion and miscommunication. Thus, Unified Numbering System (UNS) has been created for standardization and easy administration. This system is widely use in North American included Canada.

“The Unified Numbering System for Metals and Alloys (UNS) provides a means of correlating many internationally used metal and alloy numbering systems administered by societies, trade associations, and those individual users and producers of metals and alloys. It provides the uniformity necessary for efficient indexing, record keeping, data storage and retrieval, and cross-referencing.”

Above was extracted from book <<Metals & Alloys in the Unified Numbering Systems >>. This book (in CD) provides information on :

• UNS number
• Description
• Common trade names and alloy designations
• Cross-reference organization
• Cross-reference specifications
• Chemical composition

The UNS is managed jointly by the American Society for Testing and Materials (ASTM) and the Society of Automotive Engineers (SAE).

The UNS number (for "Unified Numbering System for Metals and Alloys") is a systematic approach where each metal is designated by a LETTER followed by five NUMBERS. The number is unique and composition-based of commercial materials. It is used for material reference but it does not guarantee any performance specifications and/or exact composition.

Following are overview of common commercial metals / alloys using UNS system :

• Axxxxx - Aluminium Alloys
• Cxxxxx - Copper Alloys, including Brass and Bronze
• Fxxxxx - Iron, including Ductile Irons and Cast Irons
• Gxxxxx - Carbon and Alloy Steels
• Hxxxxx - Steels - AISI H Steels
• Jxxxxx - Steels - Cast
• Kxxxxx - Steels, including Maraging, Stainless, HSLA, Iron-Base Superalloys
• L5xxxx - Lead Alloys, including Babbit Alloys and Solders
• M1xxxx - Magnesium Alloys
• Nxxxxx - Nickel Alloys
• Rxxxxx - Refractory Alloys
• R03xxx- Molybdenum Alloys
• R04xxx- Niobium (Columbium) Alloys
• R05xxx- Tantalum Alloys
• R3xxxx- Cobalt Alloys
• R5xxxx- Titanium Alloys
• R6xxxx- Zirconium Alloys
• Sxxxxx - Stainless Steels, including Precipitation Hardening and Iron-Based Superalloys
• Txxxxx - Tool Steels
• Zxxxxx - Zinc Alloys
  
  
  Typical examples :

More photos for CSCC

Additional CSCC photos...

CSCC occured on insulated vessel

CSCC occured on insulated vessel

JoeWong

Snapshot of Metal Cracks Due to Chloride Stress Corrosion Cracking

Some snapshot of metal cracks as a results of Chloride Stress Corrosion Cracking…...

Inter granular SCC of an Inconel heat exchanger tube
Source : Corrosion Doctor

Trans granular SCC of 316 stainless steel chemical processing piping system
Source : Corrosion Doctor

Inter granular SCC of a pipe
Source : The National Physical Laboratory

JoeWong

Chloride Stress Corrosion Cracking of SS304, SS316, DSS & Super DSS and Use correct MOC for seawater service

Chloride stress - corrosion cracking (CSCC) is initiation and propagation of cracks in a metal or alloy under tensile stresses and a corrosive environment contains Chloride compounds. Once the crack is initiated, it will propagate rapidly and potentially lead to catastrophic failure.

Factors that influence the rate and severity of cracking include

· chloride content
· oxygen content
· temperature
· stress level
· pH value of an aqueous solution

It has been established that oxygen is required for chloride cracking to occur. However, recent findings showed that CSCC can occur in Duplex Stainless Steel (DSS) at high chloride concentration and NO oxygen environments (HSE report 129).

The severity of cracking increases with temperature. Figure below shows several Stainless Steel materials increases it susceptibility to CSCC as temperature is increased.

Source : Sandvik Material Technology

SAF 2205 (UNS 31803) = Duplex Stainless Steel
SAF 2507 (UNS 32750) = Super Duplex Stainless Steel

CASE STUDIES

Hot gas (Shell) is cooled by seawater (Tube) from 220 degC to 180 degC in a Shell & Tube heat exchanger. Seawater is being heated from 30 degC to 35 degC and return to sea. The Shell and Tube material of construction are Carbon steel (CS) and Duplex Stainless Steel (DSS) respectively. After 2 months in operation, cracks occurred at the tube (DSS) and leads to major platform shutdown. Investigation found crack was caused by CSCC at tube. Why a CSCC occurred at DSS tube although the seawater temperature only 35 degC maximum ?

One shall understand that although the inlet and outlet temperature are below 150 degC, thermal designer may design the heat exchanger with high heat flux in order to reduce the heat exchanger area and this result tube skin temperature exceed 150 degC. Condition with Seawater which contains ~20,000 mg/l Chloride, high in dissolved oxygen, slightly acidic and skin temperature exceeded 150 degC is perfect combination for CSCC to occur for DSS. One shall check skin temperature profile especially for low flow condition or specify better material i.e. Super DSS for above service.

Comments are Welcome !

JoeWong