Dear P Dean,

The conveying air quality for the materials, which you mention (cement, fly ash, barite, and bentonite) is never questioned as normally the ambient air is used.

However, it is worthwhile to consider the eventual consequences of using “not suited” conveying air.

The content of particles in the aspirated conveying air is negligible as the convey air is filtered to a significant degree of cleanliness for protecting the compressor against wear.

I know that compressors for cement conveying installations are sometimes operating in very dusting environments, but still, the intake filters limit excessive wear and possible damage. (Also for the driving diesel engines)

Condensation of water in the conveying air is a bit more complex.

The convey-air conditions to be considered are :

- absolute pressure

- temperature

- relative humidity (RH)

There are 4 locations to consider the conditions of the convey-air.

Location 1)

Intake conditions of the compressor. In the worst case ,the RH should be considered as 100%.

Location 2)

Pressurised air at the outlet of the compressor. Here the pressure and the temperature are increased, whereby the RH decreases or stays 100% with or without condensation.

In case an air cooler is used ,this should also be taken into account .

Location 3)

At this location the convey-air and the cement are mixed at the compressed-air -pressure.

The heat-content of air and cement is equalized over both, resulting in a mixture-temperature.

Due to the high heat content of the cement, the mixture temperature at the mixing point is always relatively close to the cement temperature.

Location 4)

At this location the air has completely expanded again to atmospheric pressure and the condition can vary between the original conditions and those determined by the circumstances in the pipeline such as warmer by cement ,cooler by heat-exchange with the surroundings ,condensed water reacted

with cement ,vapour-water reacted with the cement , reaction speed ,residence time in the convey line, etc

At any location in the pneumatic conveying system, the air conditions (-absolute pressure, temperature, relative humidity (RH)) have to be calculated and eventual condensation to be determined.

Generally, the amount of cement in relation to the amount of air (Solid Air Ratio) is rather high.

Therefore, the amount of possible condensed water stays always low compared to the cement mass.

Consequently, the bonded mass of cement is also very low.

(SLR, pressure, air temperature, material temperature, cooling in the conveying pipe are all parameters, determining how much and where in the pipeline, condensation will occur)

The general reaction of cement with water can be represented by :

(CaO)3 SiO2 + 3 H2O ---> (CaO)2 SiO2.H2O + Ca(OH)2

Molecular mass :

Cement :

3 * Ca = 3 * 40 = 120

3 * O = 3 * 14 = 48

1 * Si = 1 * 28 = 28

2 * O = 2 * 16 = 32

----

228

Water :

3 * 2 * H = 3 * 2 * 1 = 6

3 * O = 3 * 14 = 48

----

54

Resulting in :

54 kg of water reacts with 228 kg of cement

(1 kg of water reacts with 4.22 kg of cement)

I have designed, built and operated quite a number of pneumatic cement unloaders and only a few had a sort of drying system (mostly a cooler with a water drain) and none of them caused any trouble with condensation as long as there were no fluidisation cloths involved and the conveying pressures were kept below 2.5 to 3.0 bar .

Even in warm and humid environments.

For these pressures, normally oil free screw compressors with internal compression are used and without cooling.

For higher pressures (up to 4.5 to 5 bar), oil filled screw compressors are used which dry the compressed air to a dew point of 3 degrC.

As these compressors do not require oil adding during many, many hours of operation, the oil cleanliness of the conveying air must be considered OK.

(It always amazes me how well the oil separators function)

Your concerns about cement hydration, plugging and 'rocks' is understandable but in practice experienced as “not an issue”.

In case you are planning pneumatic conveying installations for cement, fly ash, barite, and bentonite, visit:

www.carlsengroup.com

Have a nice day

Teus ■

Dear Mr P.Dean,

It is always a pleasure to hear that someone is enjoying this forum.

I understand (assume) that the installations are related to the onshore and offshore industry (oil fields).

Apart from cement, there is also bentonite and barite delivered by transporters using pneumatic conveying systems.

Production Supply Vessels normally are equipped with high pressure conveying systems and use oil filled screw compressors for conveying air with an integrated refrigeration drier at a dew point temperature of 3 degrC.

These installations (designed for up to 6 bar) operate normally at approx. 4.5 bar.

The high pressure in combination with ambient, or just above ambient temperatures will cause water condensation. (Especially in the tropics, where the water content in the air can be very high (x kg water/kg dry air))

The class (3) air quality deals with:

-number of particles of a certain size.

When the intake of the compressor is in a pump room, where people are allowed to stay without breathing protection, the number of particles must be considered OK.

Moreover, the compressor intake air is filtered to protect the compressor from wearing and to prevent contamination of the recirculating oil.

-Oil carry over.

The oil carry over can be given by the compressor supplier and has to be agreed by the cement technologist.

Stringent requirements ask for better oil separators.

Oil free screw compressors are also an alternative, although for the higher pressures (above 3.5 bar) they need to be arranged in double stage.

-Water content and condensation.

Drying the air in a tropical environment to a dew point of 3 degrC (refrigeration drying) will prevent condensation of water along the whole pipeline.

In the Arctic’s, in freezing conditions, adsorption dryers are a must to prevent the condensation of water, which can also form ice blockages.

From the above, it can be concluded that the water condensation issue is the most complex issue.

A dew point is not decisive, as there is difference between the tropics and the Arctic’s.

(And also the cement temperature has an influence).

The conveying pressure is also of influence on the possible water condensation.

The conveying length is important.

A short conveying length results in a less temperature drop, due to the short time available for cooling through the conveying pipe wall. On the other hand, the pressure drops in a short time below the condensation pressure at the actual condensation temperature and condensation does not occur.

The water condensation problem should be addressed per project, where the company, supplying the cement with their installation should prove by calculations that condensation is avoided as demanded by the ordering party.

(As receiving party, you are not an expert in pneumatic conveying)

For offshore cement conveying:



www.carlsengroup.com



If you like to have some example installations calculated, you can contact me through the “private message” feature of this forum.

Have a nice day

Teus ■

Dear Mr P.Dean,

Class 3 :

particles = 5 micron

dew point = -20 degrC

oil content = 1 mg/m3

Class4 :

particles = 15 micron

dew point = +3 degrC

oil content = 5 mg/m3

(Dew point - 40 degrC is Class 2)

The majority of pneumatic conveying installations with oil filled compressors for conveying air, deliver an air quality of Class 343.

3 - particles = 5 micron

4 - dew point = +3 degrC

3 -oil content = 1 mg/m3

By requiring a Class 3 dew point (-20 degrC), you might rule out a whole fleet of supply vessels and certainly the low pressure systems (at approx. 2.5 bar to 3 bar), which are operating with oil free compressors and no drying at all.

The worldwide application of Class 343 indicates that this should be OK.

Only for the Artic conditions, I would be more carefull.

Success

Teus ■

Dear P Dean,

For sure you would have eliminated many of the plants Y've not seen too.

If I can help you with an Excel sheet for the calculation of air conditions and condensation, you can contact me through the private message function of the forum.

Take care

Teus ■