Yearly Archives: 2013

In my latest article about the different materials used for chemical pumps I wrote a brief guide helpful for a first materials selection according to their properties and characteristics. Now I would like to analyze each material from the point of view of their typical applications.

Polypropylene (PP):

Mechanical applications: PP is used for mechanical parts in corrosive environments. Compared to polyethylene with a high molecular weight, polypropylene has a higher traction resistance.

Alimentary applications: physiologically inert when it has a natural color, polypropylene is suitable for  use in contact with foods.

Electrical applications: good dielectric characteristics and weather-proof.

Chemical applications: PP is widely used in chemical industries because of its high resistance to acids and alkali. Thanks to its higher resistance to temperature than PVC  it’s used for valves, flanges gears etc. in chemical, galvanic and petrochemical industry. It’s not suitable for use with high concentration oxidizing acids.

PVDF:

Chemical applications: the high chemical resistance to acids and alkali is typical of fluorinate polymers (PVDF) that’s why it’s mainly used for parts in petrochemical and chemical industries.

Alimentary applications: physiologically inert when it has a natural color, it’s approved by different organizations for use in contact with foods. It’s often used because of this characteristics, especially in alimentary machines and in pumps for alimentary liquids.

Electrical applications: good dielectric characteristics, self-extinguishing without adding halogens and weather-proof

Mechanical applications: the low friction coefficient makes is suitable for bearings even if they work in water.

PTFE

Mechanical applications: the low coefficient of friction makes it useful for bearings unless they have to support a great weight.

Alimentary applications:  physiologically inert, it’s approved for use in contact with food by some organizations, but in some countries it’s questioned the possible use in contact with food.

Electrical applications: thanks to the good dielectric characteristics, the self-extinguishing and the stability to bad weather it’s used more and more in this field.

Chemical applications:  typical of fluorinate polymers is the high chemical resistance to acids and alkali. It’s main use is for components in petrochemical and chemical industry.

EPDM

Mechanical applications: Gaskets, section bars and technical items for cars, handmade articles for anti-acid protections.

AISI316

Suitable for plants for production of nitric and sulphuric acid and for the relative equipment (pumps, valves and pipes). It’s used in very aggressive environments (textile industry, paper industry, oenological- alimentary  industry, and especially naval industry).

When dealing with acids it’s important to use suitable chemical pump materials resistant to corrosion which are highly efficient and require minimum maintenance. That’s why, after years of experience in chemical pumps field, we decided to use Polypropylene, PVDF and AISI316 as standard materials for our pumps and EPDM, Viton and PTFE for the o-rings.

Every material has its own properties and characteristics that make it suitable for pumping different acids because, as I wrote in my previous article about pump selection, it’s really important to choose the right pump material for every chemical application.

 Polypropylene (PP)

1. Polypropylene is well known with the name Moplen (™)
2. It’s a thermoplastic material extremely resistant to many solvents, bases and acids such as for example acetone, caustic soda and chloridric acid.
3. It’s not suitable for concentrated, oxidizing acids.
4. It’s used in many different industries for parts subjected to relatively modest forces.
5. Temperature range for PP pumps is from 4° to 60°C.

Polyvinylidene fluoride (PVDF)

1. It’s a thermoplastic material of the fluoropolymer family and it’s used generally in special applications requiring the highest purity, strength, and resistance to solvents, acids, saline solutions, to alkali and bases.
2. It’s mainly used with liquids such as for example sodium hypochlorite, concentrated sulfuric acid, nitric acid, gasoline.

3. It doesn’t undergo deformation under loads and has a high mechanical resistance.

4. Temperature range for PVDF pumps is from -40° to 90°C.

 Stainless steel AISI316

1. It’s a metallic material used when both the properties of steel and the resistance to corrosion are required.
2. Particularly indicated for pumps working with high temperature liquids, oils, kerosene, alcohols.
3. It’s used to pump acids up to a maximum temperature of 160°C

Also the o-rings are in contact with the liquid and their material is very important to avoid leakage.

EPDM

1. It’s an elastomer with a good resistance to heat, ketones, ordinary diluted acids and alkalines.

2. Temperature range: from -40°C to 140° C

3. We usually use EPDM o-rings as standard with PP pumps

VITON

1. It’s a fluoropolymer elastomer with a good resistance to hydrocarbons, acids, fuels, mineral and vegetable oils but it’s incompatible with ketones and organic acids
2. Temperature up to 150°C
3. We usually use VITON as standard with PVDF and AISI316 pumps

PTFE

1. Polytetrafluoroethylene known also as Teflon (™).
2. It’s highly resistant to the attack of acids and basis  in corrosive environments but it has poor mechanical properties such as traction and compression.
3. T

   emperature range: from 4°C to 260°C

These are general rules that can be useful for chosing pump material but it’s better to consult a good chemical compatibility chart to select the best material for each chemical application.

When operating in a potentially explosive atmosphere it’s necessary to use a suitable pump designed and manufactured according to the ATEX directive 2014/34/EU. Here are five main things you should know when dealing with Atex Pumps:

1. Equipment used in potentially explosive atmospheres is divided into groups and categories according to the degree of protection offered. You can see and example in the table below.
   

   Table 1 – Atex groups and categories

2. Pumps for use in potentially explosive atmospheres will normally be classified under Group II, Categories 2 and 3. It is the responsibility of the end user to classify the zone and the corresponding group (dust or gas) in accordance with the EC – Directive 1992/92/EC. Once the pump manufacturer knows the Atex classification needed by the customer, it’s possible to select the right pump solution.
3. Atex pumps have a special design which prevents the formation of sparks and the ignition of explosive atmospheres that may be produced or released by equipment. They must be assembled to Explosion proof motors with the same safety measures.
4. Atex pumps for zone 1, where explosive atmosphere caused by gases, vapours, mists or air/dust mixtures are likely to occur, must be equipped with safety devices like dry running protection and thermoprobe PT 100. The dry running protection checks constantly the active power of the motor, which is the medium value of the instantaneous power absorbed by the pump, and prevents the dry-running of the pump, the closed discharge and the blocked suction. PT100, instead, is a resistance temperature detector used is used for monitoring the operating temperature of the pump.
5. Atex pumps need a certification issued by the manufacturer and shall be marked with the following information:
   

   Atex mark

• name and address of the manufacturer
• CE marking
• series type, year of construction and serial numer
• the specific marking of explosion protection followed by the symbol of the equipment group and category

The European Commission issued a guideline to facilitate the application of Atex directive 2014/34/EU which could be useful for the first approach to Atex products.

One of the important aspects to keep in mind when installing a pump in a plant is NPSH (net positive suction head).  There are two values of NPSH: NPSH available calculated by the plant engineer, and NPSH required which depends on the characteristics and performances of the pump. So why are these data so important?

NPSH available

The term is normally shortened to the acronym NPSHa where the ‘a’ denotes ‘available’. It is the result of the absolute pressure of a fluid at the inlet to a pump minus the vapour pressure of the liquid.

NPSH required

This is a term used by pump manufactures to describe the energy losses that occur within many pumps as the fluid volume is allowed to expand within the pump body. This energy loss is expressed as a head of fluid and is described as NPSHr (Net Positive Suction Head requirement) where the ‘r’ suffix is used to denote the value is a requirement. Different pumps will have different NPSH requirements depending on the impeller design, the impeller diameter, the flow rate, the pump speed and other factors. A pump performance curve will usually include a NPSHr graph so that the NPSHr for the operating condition can be established.

Avoiding cavitation

The essencial condition to have a good pump functioning is that NPSHa is higher than NPSHr to avoid cavitation.

Cavitation is the formation of gas bubbles when the pressure within a fluid falls below its vapour pressure. If a fluid which contains gas bubbles is allowed to move through a pump, it is likely that the pump will increase the pressure within the fluid so that the gas bubbles collapse. This will occur within the pump and reduce the flow of delivered fluid. The collapse of the gas bubbles may cause vibrations which could result in damage to the pipework system or the pump. This effect is known as cavitation.

Correct pump selection

In order to choose the correct pump size, it’s always important to know the NPSHa and check if it is suitable with the NPSHr of the pump. Once the relationship between these values is right the pump will work correctly and cavitation will not occur.