8 replies to this topic

[suryatej001]

In our industry to cool lube oil shell & tube heat exchangers are using.

Shell side: Lube oil

Tube side: Sea water

Tube Material: Cu alloy (exact alloy name not known for us)

These tubes are prone to scaling.

1.How to avoid scaling?

2. what are the methods for descaling ?

3. which chemicals can be used to remove these scales in chemical cleaning?

[Art Montemayor]

Are you a student?  This is not a typical student problem; it is a field engineering problem that does not rely on academic and analytical methods to resolve.  It is an industrial, practical problem that depends on experience or expertise in the specific application.

 

Firstly, copper is not compatible with seawater; few materials are.  You could have a copper alloy that is called “Admiralty” metal that was developed by the British a long time ago for use on sea-faring ships.  Cleaning out the scaling requires basic hand-applied tools and effort.  Therefore, it depends on WHAT TYPE of heat exchanger you have.  You normally would apply manual cleaning in the form of a rotating bronze brush.  You wouldn’t use a steel brush because the steel would remove the softer copper alloy.  Acids also cannot be normally applied because they would attach the copper alloy.

 

We don’t know what type of “scales” you are referring to.  Normally, scaling does not take place except when you run the seawater too hot and it deposits its solid contents.  The total answer may lie in you running more seawater flow rate through the cooler.

 

 

[Atttyub194]

Dear suryatej001 & Dear Art Montemayor

 

Good day

 

I will give a detailed response on weekend on the subject

 

Hope it will be fine

 

Best regards

[suryatej001]

@ Art Montemayor

 

Thank you for your reply... The problem is, the shell and tube type lub oil coolers which we use, need to be frequently cleaned manually (which we call as Cane cleaning) due to drop in temp difference of the lub oil inlet and outlet. And it is a labourious process. The Cu alloy which we use for the tube material is suspected to be 70-30 Cu-Ni or 90-10 Cu-Ni. And yes its absolutely sea water which we use as a cooling medium and hence we also encounter problems of organic growth like barnacles, etc. But we are most bothered about the scaling thats happening in the tube side due to sea water constituents. Also we do chemical cleaning of these coolers every 6 months using a chemical whose composition is phosphoric acid+surfactant (wetting agent)+corrosion inhibitor. And this seems to be not much effective method since the temperature difference drops down in a span of say 30-45 days. So I wanted to know whether there are any other chemicals which can be used in the place of the one which we are using right now and being more effective than that. What about Sulfamic Acid???

[Simonpen2001]

Do you also have any knowledge or documentation about scaling/descaling in shell and tube geothermal power plant?

[Atttyub194]

Dear All

 

Good day !

 

The problem what I perceived is more than effective method of chemical cleaning. Additionally, basic relevant data which is required to pin point the problem is missing. I have observed that people are reluctant to provide necessary data on forum which might help other experienced people to give relevant comment and share their experience. This to some extent reduces the effectiveness of the forum

I am giving you a detailed reply with some suggestion and assuming that you will keep all the members posted on the forum the result and remedy of your issue
Let starts with the basics
 

Sea water is used as heat sink by three possible routes
 

1. Direct Use of Sea Water

• The seawater is pumped directly to the heat exchanger and there is direct transfer of heat from the hot medium being cooled to the seawater

1. Indirect Use of Seawater
   • Heat is exchanged between the seawater and a secondary stream which could be fresh water; or a mixture of fresh water and, or , glycol. This secondary stream is then used as cooling medium. In general, for offshore use, water with typically 30% of tri-ethylene glycol (TEG) or mono-ethylene glycol (MEG) is used as the secondary cooling medium. For onshore use, fresh water is often used if climate conditions permit.
2. Use of Seawater in Cooling Towers
   • In some areas, it may be economical to utilize seawater in cooling towers. In this case  the heat rejection is not only to the sea but also to the atmosphere due to partial evaporation of the seawater. This obviously can make available an alternative source of cooling tower water where fresh water is expensive. It can also lead to a considerable reduction of pumping power and thermal pollution
   • Usually, in such systems, the seawater is cycled through the cooling tower for no more than two cycles, to minimize corrosion and fouling. For fresh water is concentration cycle usually varies between 3-5 depending on the chemistry of water and metallurgy of the downstream plant

 

Needless to say, the information that what type of above system in used help in establishing the type of fouling

The next basic information, which is missing in your case is the type of fouling. Let us discuss the most common type of fouling in sea water, so that we may try to pin point the fouling in your case

1. Crystallization Fouling.

This type of fouling occur when deposition of calcium carbonate, calcium sulphate and other salts occur due to increase in temperature as these salts have a solubility in water which diminishes with increasing temperature. Thus leading to crystallization of deposits of the salts on the heat exchanger tubes and reducing its performance

1. Corrosion fouling:

Although this type of fouling is rare in lube oil cooler, but it does not means that it would not occur for lube oil coolers. In this case some metals are oxidized to produce insulating layers of oxides on the tubes

1. Biological fouling

In sea water Service exchanger the most common type is Biological Fouling. In this case a whole range of biological growths form on heat exchanger tubes in seawater. The species attached range from micro-organisms (bacteria, algae) to macro organisms (mussels, barnacles, etc

Proper identification of biological range is necessary for the selection of biocide

1. Particulate fouling.

Sea water may contain many types of silt, mud, sand or other finely divided particles that may settle on the heat exchanger surfaces and act as an insulating layer. This type of Fouling is called particulate fouling

You should establish what type of fouling is occurring in your case as this is deemed mandatory to suggest remedy

Fouling may be predominant in sea water system due to following factors<ul>
 

1. Surface temperature

The temperature at the interface between the seawater and the solid surface is of crucial importance. For crystalline fouling, the deposition is small at temperatures below 60oC but can become serious at elevated temperatures

Please do check the out let temperature of your exchanger. This might be one of the cause of rapid fouling. I have seen crystallization fouling over a period of a month for steam condensers with outlet temperature operating with water outlet temperature of 65 Deg C

Please see attached file for impact of temperature on different type of fouling
 

1. Bulk temperature:

It may be noted that Biological growth becomes faster as bulk temperature increases from ambient values. At high bulk temperatures the organisms may be killed; however, long residence times may be required at elevated bulk temperatures, to kill the organisms present

 

1. Water velocity:

Water velocity generally has only a small effect on crystalline fouling at lower velocities but may tend to shear off the crystalline deposits at higher velocities.

 

Biological fouling rate tend to increase with velocity at low velocities, reflecting the need to supply nutrients to the surface which needs at least a low flow rate to sustain mass transfer. However, the risk tend to reduce at high velocities. This is the main reason that water treatment supplier recommends using water velocities in the range of 1.5-2.0 m/sec

 

Please check water velocities as in most cases I have seen the problem occur due to low velocities through exchanger and due to use of high over design factor at design phase

 

1. Solids.

 

In seawater, sand is the most prevalent solid and erosion may be expected for sand particles with sizes greater that around 50 micrometres at concentrations no more than a few hundred ppm.

 

It is pertinent to mention that Titanium and Superduplex alloys are much less prone to erosion-corrosion than copper-based alloys.

 

I am not going into the details of most suitable material for sea water service for your application as the material you have indicated is used on many applications

 

Coming back to the remedy please do consult a water treatment company for suggestion of proper biocide or shock polymeric dozing at the inlet of exchanger to avoid frequent cleaning

 

You may consult http://www.merusonline.com/theproduct for supply of merus rings which may be effectively used for your specific case. I my self donot have any experience of using such rings, however, I have heard good experience for using in different type of fouling

 

Regarding chemical cleaning, it may be noted that without knowing the type of scale you can not choose an anti scalant and a corrosion inhibitor. However, you may find good guide lines in ASM book  Corrosion in Petrochemical Industries

 

To my best of knowledge sulfamic acid is not a very good chemical cleaning agent . Instead 2-10  % HCl may be tried with a good corrosion inhibitor. We have used  even 10 % HCL inhabited at 60-65 Deg C for 4 hours for CaCO3 deposits

 

You may find a number of corrosion inhibitors for cooper and alloys from following companies

1. Armohib ( http://sc.akzonobel....rmohibfinal.pdf)
2. IBIT  (http://www.asahi-che...t/cleaning.html)
3. Rodine (http://www.henkelna....odine-18842.htm)

Please do test the effectiveness of the inhibitor before implementing in the filed

 

Hope the information will help

 

Best regards and God Bless You

Attached Files

•  Fouling (Rf) with Surface Temperature.pdf   17.72KB   15 downloads
•  Fouling (Rf) with Water Velocity.pdf   18.51KB   14 downloads

Edited by Attyub194, 02 June 2013 - 11:41 AM.

[Atttyub194]

Dear Administrators

 

Good day!

 

I have to re-write the above response three times as when even I posted it appeared to be a junk

 

Please look into this matter

 

Regards

[Art Montemayor]

^Attyub194:

 

^{With you not providing all the necessary data, I can't tell what it is that the Forum Editor is doing wrong - or perhaps what you are doing that is wrong in posting.   Please furnish a detailed account of what problem you have encountered so that it can be fixed.   Up to now, I have not had any problems posting - even very lengthy and attached documents.   This matter will be looked into when we have an identification of WHAT to look into.}

[Atttyub194]

Dear Art

 

Good day

 

Thank you for your prompt response

 

I do not have any problem before yesterday. Yesterday when ever I tried to save the post it tuned into non readable junk

This is what I have. On third attempt I succeeded in posting the above thread

 

Hope this will help

 

Best regards

 

Ahmed Attyub