11 replies to this topic

[vinod kumar turki]

Dear All ,
hello,
I am new to this form.
I want to share a issue and seek expert view on it,

Expansion tank.
I am seeking the help on the above said topic.
I have received a design of water system. In this system we have two loops, close and open loop system.
Interface between the two system is a plate type heat exchanger.
Close loop system is feeding water to the plant at 50 deg C. and returns at 55 deg C.
The return water is fed to centrifugal pump with a head of 78 mtr. , flow 750 cum X 2 pumps running. The return pressure of this water is expected to come back after passing the coolers at 3 ~ 4 bars
In this system we have an expansion tank. This tank is used as a make up of close loop incase of any water loss in the system.
Water is fed to this tank through a pump and pump operation is controlled by the pressure in the supply pipe line of this tank.
There is an automatic FLOAT valve used to shut the supply of water to expansion tank ,on reaching to the required level. Once the tank level is attained, the valve shuts off and the line pressure increase which gives signal to the pressure switch and pump stops.
The discharge from this tank is taken to the suction of close loop system.
This tank is having a overflow line and a vent on the top.
Tank bottom is at a elevation of 6.5 mtr.

How can this tank work as an expansion tank?


Now my thinking, how the system should have been is as under, pl. correct me if I am wrong.

1) What I believe is that this tank should be pressurized and the pressure in side the tank should be more than the pressure of the close loop pump suction pressure. Then only it serve the purpose of making up of water loses.

2) Or the return water from the plant should have been sent in side the expansion tank and the outlet of the tank should have been fed to the close loop pump suction.

If correct me if I am wrong as far as the above two points are concerned pl correct me.

Now the piping is almost complete and the option 2 is ruled out (for my academic knowledge I want to know weather the option two was a better on or not.)

As far point 1, is concerned this can be done with minimum changes in piping.

Now my question is how make use of this system as most of the piping is done as per the attached drawing .

what should have been the best system.

regards

Attached Files

•  expansion_tank.dwl.pdf   123.64KB   200 downloads

[Art Montemayor]

Vinod:

Your diagram is very difficult to interpret. I don't know the reason for the bright colors or the unfamiliar symbols, but they are difficult to trace and to interpret the meaning. Why haven't you generated a conventional PFD or P&ID with conventional symbols? Your readers will have a difficult time trying to understand what it represents. For example, what is the parallelogram symbol in the box titled "PLANT"? the 20" line enters here and then goes nowhere.

What do you mean by the phrase "centrifugal pump with a head of 78 mtr, flow 750 cum X 2 pumps running"?

It is very difficult to make any meaningful comments or remarks when I don't understand your symbology and the way the drawing is generated. I have worked in many foreign countries and under many engineering systems and methods. But I have never seen an engineering drawing such as this one. It seems to put more emphasis on the use of check valves than on the logic of understanding it.

I think I can GUESS what is being represented, but I prefer not to guess and waste a lot of time and effort. Engineering drawings should be clear, self-explanatory, and follow conventional symbols and depiction of a process. Please generate a simple drawing showing the critical equipment and lines as well as any details that relate to your query. Once we can understand what you mean to do, we can discuss your query.

[vinod kumar turki]

DEAR MR. ART,

Thanks for your response and time.

Yes i fully agree with you on the symbols and etc. Actually I have recieved it from some supplier for one of my process.

I am now attaching a xls. file in which i tried to explain in schematic diagram. I hope it will explain the question raised by me.

regards
vinod

Attached Files

•  SCHEMATIC_DIAGRAM.xls   30KB   101 downloads

[Art Montemayor]

Vinod:

Thank you for your prompt and rapid reply.

Attached find my comments within your revised Excel Workbook.

Isn't this an efficient and accurate way to communicate engineering information?

I hope this helps you out.

 Schematic_Diagram_Rev1.xls   182KB   168 downloads

[vinod kumar turki]

Dear Arts,

Many Many Thanks,

I request you give me some tips how to make this type of scketch with the help of xls.

I tried to find in help but couldn't, request you once again for this help.

Yes , the purpose of this tank is to make up the water losses in this system, Thus my thinking was right that this tank should be totaly pressurised and must have pressure higher than the pump suction pressure.

regards
vinod

[Art Montemayor]

Vinod:

You can use our SEARCH feature with the key words "Excel draw" and find the following URL:

http://www.cheresources.com/invision/index.php?showtopic=3684&st=0#entry23095

There, you will find the thread that initiated an Excel workbook that you can download and follow as a tutor in drawing with Excel tools.

You say that the purpose of the expansion tank is to make up the water losses in this system, and you are right. That is ONE of the purposes for the tank. But be aware that there are various types of potable water losses in this system:

1. vaporization losses (if an atmospheric-vented tank is selected;
2. external leaks through packings, gaskets, fittings, and the plate and frame heat exchanger;
3. leakage into the process plant coolers (if you employ a pressurized expansion tank).

From the type of cooling you are investing in, I suspect that you may be cooling a process that is very special and that cannot tolerate process fluid contamination from ordinary cooling tower water. It may be that your process fluid is for human consumption or requires extraordinary purity levels. And for those reasons you must ensure that if the process coolers develop a leak, the direction of the leak must be from the potable cooling water INTO the process fluid. If that is the case, then I agree with your process logic and decision to pressurize the expansion tank.

Bear in mind that this decision involves more costs. You must either have high pressure municipal water available or install a pressure booster system to get the potable water into the expansion tank 100% of the time while you are operating. Additionally, you have to ensure that you always have a volume of inert gas (air or nitrogen) in your expansion tank to absorb the water expansion. You will require a water level gage and alarm to detect the level of the water in the tank.

I hope this information helps you out.

[vinod kumar turki]

Dear Arts,
Thanks once again,
the below lines i couldn't get properly,


"Additionally, you have to ensure that you always have a volume of inert gas (air or nitrogen) in your expansion tank to absorb the water expansion. You will require a water level gage and alarm to detect the level of the water in the tank."

what is reason for using the inert gas and how it can be connected to presurised tank.
why we require to absorb the water expansion.

As you have said above that i will require the high pressure muncipality water, I have the booster pump to boost.

regards
vinod

[Art Montemayor]

Vinod:

All matter, whether gas, liquid, or solid, will expand upon going through a temperature increase. When dealing with liquids, a coefficient of expansion is involved.

If you fill any liquid system 100% with liquid and leave no vapor space for it to expand when it increases in temperature, the system will exhibit a hydraulic expansion and subsequent pressure increase - sometimes enough to rupture the system if a thermal expansion device is not installed. That is why ALL liquid recirculation systems have to have a means to freely expand. An expansion tank serves that purpose - hence the name - and also serves as a means to measure and control the liquid level or inventory. I thought you would already know that.

[vinod kumar turki]

Dear Art,

Thanks, sorry to bother you once again on this issue.
The expansion theory is clear to me, and I have theoretical Knowledge about this.
To be true, I have never worked on this system and am working for the first time.
I am attaching here with a drawing which I have received for the expansion tank.
Let me try to explain what I under stand by this drawing.

As you rightly said that "Any liquid system 100% with liquid and leave no vapor space for it to expand ,when it increases in temperature, the system will exhibit a hydraulic expansion and subsequent pressure increase - sometimes enough to rupture the system if a thermal expansion device is not installed."

You can see in this tank there are two chambers in this tank. One is water side and the other is air cushion side.

Water side.
The water side when filled with water will leave almost no vapor space. The expansion can take place due to various reasons on the water side.

Air side
Here you can see there are two nozzles N3, N4 for air inlet and air out let and N5 as safety valve Nozzle. I am not able to understand the purpose of air outlet line N4. and where it has to go.

I believe air from online should be filled within this chamber, with no out let. The air pressure in this chamber must me less than the design pressure of the tank (say X) and pressure safety valve should pop off at X+ 1 Kg/cm2 which should be again 1~ 2 kg/ cm2 lesser than the test design pressure of the tank. Also the water side tank pressure and airside tank pressure should be more or less same before expansion.

Once the expansion starts in water side it will start pushing air side and results in rise in pressure in air chamber. If air crosses the set pressure of safety valve, the safety valve must pop off. Then why N4, or whole thinking of mine wrong.

Pl comment.

regards
vinod

Attached Files

•  expansion_tank.pdf   124.19KB   110 downloads

[Art Montemayor]

Vinod:

You have not shared with us a lot of basic data and scope of work about this project of yours. I have presumed that you prefer to keep this information as private – which is OK, but it limits me and others in being able to assess the validity and correctness of the engineering involved. I still don't have a response as to why you require a pressurized potable water cooling system. I only guessed at the reason(s) – but never got a response. I hope you have strong and sound reasons for designing a pressurized cooling system because it looks like a very expensive one.

I believe I know what is being designed (I just don't know why). Who is the designer and engineer on this project and what are their calculations based on? What are the specifications for the basic design? You have some very valid and probing questions regarding the design – I have even more! Who did you commission to do the engineering and why aren't you addressing the person who designed and engineered this tank for you? That would seem to be the most direct and quickest way to resolve the issues being raised. My comments and responses to your post are:

1. The "expansion tank" is, in reality, a storage tank. It is designed to hold 10 tonnes of water! Although expansion tanks can be designed to serve as storage tanks as well, this is a huge amount of water and there must be a good reason for this. This involves a large amount of extra capital expense – especially when you design it as a horizontal tank with ellipsoidal heads and with a design pressure of 12 kg/cm2.

2. A 12 mm shell thickness seems rather thick and normally would be considered an "overkill" for a simple water expansion tank. Moreover, the use of full, vessel flanges to simply serve as a rubber diaphragm fixture makes the design very expensive and, in my opinion, impractical to build and to operate. The use of SA 516 Grade 70 steel plate is great – however, it is top, premium quality pressure vessel steel plate that is expensive. A 500 mm manway is considered much too small for this size of tank. Safety reasons dictate a larger size for confined space entry by a human being.

3. How do you intend to introduce makeup water if you don't have a level detection device and nozzles on the tank? Obviously the tank is designed to operate 100% full. If so, how is makeup water need to be detected and introduced? I have to presume you will use pressure as the indicator of water makeup need.

4. What I see is a type of bladder expansion tank – which works OK, but normally it should be oriented in the VERTICAL position. The horizontal position for a bladder tank makes it very difficult to design and operate.

5. You must obtain an MAWP (Maximum Allowable Working Pressure) for this tank if you insist on it being a pressure vessel fabricated under ASME Section VIII. This MAWP value is the basis for the setting on the pressure relief valve that is REQUIRED on the vessel. This PSV should be protecting the WATER side of the vessel – which is exposed to possible process pressure from the process coolers (a tube rupture) or a pool fire around it. The air side of the bladder might also require a PSV; I need to have a full process description to make these kind of judgments. We still know nothing about your process or scope of work.

6) I do not agree with mounting the bladder on the side of the vessel. If you must have a bladder type of expansion tank, then the top manway seems like a much better and practical location to place a diaphragm. It would certainly reduce the capital cost of the vessel. A full vessel flange is ridiculous in my opinion.

[djack77494]

Wow, this really is unusual. Art has done an excellent job of raising concerns about your design. I have never seen a design like this, and must question the intent of this design. NORMALLY, the expansion tank is a vertical low pressure (or even atmospheric) vessel located at the high point of the system. It is sometimes quite small since the reuqired volume is just that needed to absorb the fluid's expansion and contraction cycles in heating/cooling between minimum and maximum temperatures. That volume isn't much. I would say that there must be ulterior motives in this design which confound attempts to explain what is happening.

[vinod kumar turki]

Dear All,

Give me some time & I will explain it to you.

It is a mess created by some one and I am trying to clean it up with your help.
Art has rightly said this huge system is not required.

regards