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balerjoe
02-09-2005, 07:58 PM
Hello everyone I'm new to this forum. I work at a Milk plant here in the US. I have a question here if any can help. Through the press our milk pressure is 46psi. Glycol is unkown I'm told it fluxuates depending on load.
My (in house engineer) told me that glycol & milk pressure dont have to be close. For example: Milk 46psi Glycol 10psi. Would work perfectly fine w/no leaks.
I just got done visiting w/a refrigeration engineer. He informed me that they should be within 2psi of each other. With the product always being higher than the glycol. This we do but not within 2psi of each other. Now will having glycol at 20psi & milk at 46psi cause the Plate press to leak milk into glycol?
Thanks guys

botrous
02-09-2005, 09:05 PM
If any microscopic fissure exists there will be a leak . . . . to be in the safe side , let the pressure be very close with 1 or 2 psi in addition for the milk pressure , this way , if there is any microscopic holes the milk will go to the glycol side not the glycol side to the milk , which may result in a damage of the whole recipient , now if you are sure that there are in no way any probable leak holes or fissures , then pressure doesn't matter , but if in any case there is one than let there be a pressure difference , with the milk having the highest one.

Best Regards

US Iceman
03-09-2005, 05:27 AM
I agree with the concept of the milk pressure being slightly higher to prevent the glycol from contaminating the milk if the press plates leak.

However, the two psi differential (in favor of the milk pressure) is a new one for me. What is the logic for a maximum 2 psi differential?

If the glycol pressure fluctuates, it sounds like your glycol system may be reaching the deadhead pressure of the pump (little flow, but higher pressure). This could occur if the glycol control valves are all shutting off on the process exchangers (no cooling required).

When the control valves open, the pump head decreases, but the pump flow rate increases. I'm assuming the pump is a centrifugal pump.

If this is the case, the pump discharge pressure on the glycol could conceivably achieve a higher pressure than the milk pressure in the press plates. The amount of pressure that can be created in this situation is dependent on the pump curve.

This is based on the commonly seen pumping system using a constant volume system. Some of this behavior can be mitigated by using a variable volume system (VFD on the glycol pump).

The control valve type and location should be evaluated to ensure the proper cooling of the process.

Any potential for leaks is dependent on the plate gaskets, gasket material for compatibility, and design pressure.

I would be really interested in hearig the explanation for the 2 psi differential requirement.

botrous
03-09-2005, 10:37 AM
Well the explanation of the 2 psi differential pressure requirements comes from here :
For continuous pasteurizing, it is important to maintain a higher pressure on the pasteurized side of the heat exchanger. By keeping the pasteurized milk at least 1 psi higher than raw milk in regenerator, it prevents contamination of pasteurized milk with raw milk in event that a pin-hole leak develops in thin stainless steel plates. This pressure differential is maintained using a timing pump in simple systems, and differential pressure controllers and back pressure flow regulators at the chilled pasteurization outlet in more complex systems. The position of the timing pump is crucial so that there is suction on the raw regenerator side and pushes milk under pressure through pasteurized regenerator. There are several other factors involved in maintaining the pressure differential:
• The balance tank overflow level must be less than the level of lowest milk passage in the regenerator
• Properly installed booster pump is all that is permitted between balance tank and raw regenerator
• No pump after pasteurized milk outlet to vacuum breaker
• There must be greater than a 12 inch vertical rise to the vacuum breaker
• The raw regenerator drains freely to balance tank at shut-down

Reference : http://www.foodsci.uoguelph.ca/dairyedu/pasteurization.html
This theory can sure be applicable to the application we are talking about .

Best Regards

US Iceman
03-09-2005, 06:16 PM
The website Botrous provided is very interesting.

The pressure differential issue is still one of preventing or reducing cross contamination of the two fluid streams.

This I can follow.... However, why is only a 1 or 2 psi differential required? I cannot think of a reason why a slightly higher pressure would cause any concern.

What I see is the need to maintain a positive/higher pressure over the the opposite fluid stream.

The system described in the website allows the differential pressure to be maintained in all operational concerns from what I can tell.

Am I missing something here???

botrous
03-09-2005, 06:41 PM
Hi US Iceman , how are you doing ?
Well my own interpretation for the 1 or 2 psi differential pressure .
1) it's suffisant enough to if a microscopic hole exists the product will not be contaminated , i mean the direction of leak will be from the product to the cooling or heating agent.
2) if the pressure difference become larger , let's say 10 psi , the same microscopic hole will leak propotionaly to the pressure difference , mean the leakage will increase .

example : if you have a pressurized water bottle and you make a pin hole in it's buttom it will be leaking in a very high rate comparing it to a bottle of the same size that isn't pressurized or have a pressure slightly higher than the atmospheric pressure.

Cheers
Best Regards

US Iceman
03-09-2005, 08:05 PM
Hi Botrous,

I'm fine, thanks.

What you say makes sense, lower dP across a hole or orifice does decrease the mass flow rate. I can agree that this would limit the cross contamination.

However, is it necessary to create special pumping systems and controls to try to limit or control the dP?

Perhaps this can be done by careful selection of the glycol pumping, piping, and control system??

A variable volume pumping system with the appropriate control valves and piping might make a better system design. A pumping system using a variable frequency drive controlled by a pressure transducer in the glycol system would allow reasonable control of the dP.

Some of my thoughts on the subject.

If there are regulations or codes that say this is required then we have to follow it. If someone says this is required, my question to them is: where is this written down and who requires it, and why?

Some things have a way of getting started because someone said so. I'm a little skeptical and say show me.:o

Thanks for the link you provided. I will review it later.

US Iceman

botrous
03-09-2005, 08:26 PM
I agree with every world you said Us Iceman .
I searched for codes and regulations but never found what i'm searching for , so if you found something i'll be glad if you let me know .
And yes i agree the dp control is more complex than words to say , but when the system requires it (some very basic controls are listed in the website ) , that's to engineers to find out a solution . . . that's how we make our days no ?
I have another idea to control the dp , but i think it will contaminate the product , i will do a little search about that and hopefully let you know the results.

Best regards

US Iceman
03-09-2005, 08:40 PM
I think this could get into a very in-depth discussion on pumping systems, piping, and control methods.:eek:

Too many things we do not know about the system in question.

As you say, this is how we make our wages and spend our days.

Pumping systems and the various methods of installing these is very interesting. It is always fun to see what other people do!:rolleyes:

Have a good weekend. I have to run.

Regards,
US Iceman

botrous
03-09-2005, 08:54 PM
Yes it could get us into a very deep discussion , but it always improves our way of seeing things no ?

But the funny and weird thing man is that we never heard till now from the one who started this thread . . . :) ((((Balerjoe if you read this post say something so we know you are still living ))))

Thanks have a good weekend yourself , i gonna spend my sunday in the mountains , hope you will have fun as much or more than i do .

Best regards

balerjoe
06-09-2005, 02:47 PM
Hey guys I've been out of town. Doing some camping over the holiday.
This question came to my mind as we are having contamination problems.
It stands to reason they both need to balance each other out. All the engineered drawing I've seen show a 1psi to 2psi diffrence. Any further evidence that this is legit is most helpful.
Thanks
Joe

US Iceman
06-09-2005, 04:09 PM
Hi Joe,

If you are experiencing contamination problems, then you either a have a momentary or sporadic leak, or a very small continuous leak.

Is the glycol getting into the milk? You said in your first post the glycol pressure was unknown.

I'll ask the obvious questions first.

Are the plate gaskets in good condition?

Is the pressure rating of the gasket material higher than any potential pressures that can be developed by the pumps (milk or glycol).

Beyond this, I am not sure why the expected 1-2 psi difference is being specified, unless it is a limit of the press plate design.

On the other hand, the small differential may be specified to limit the amount of contamination if a leak does develop.

Have you called the manufacturer and asked them about this? I suggest you start with them.

There has to be a reason, and I would also like to know.

Regards,
US Iceman

balerjoe
06-09-2005, 05:07 PM
The gaskets were in poor shape. But since have been replaced. I was inquiring if any on the board have experience w/this. I believe it was in fact the gaskets. But it was brought to my attention that 2psi is the norm. Just checking
thanks
Joe

Argus
06-09-2005, 06:19 PM
One small reason to be thankful for lactose intolerance.
________
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botrous
06-09-2005, 09:22 PM
Hi Balerjoe and welcome back, the 2 psi pressure difference is sufficient to keep a one way contamination , i mean that the milk contaminates the glycol for micro holes , but for big holes the contamination will be in both ways , because the larger the hole is , the biggler the turbulance effect between the two liquids will be , and the contamination will occure in both ways but sure not in same proportions.

After the gaskets beiing changed , you still have leaks ? and in which way ?

balerjoe
06-09-2005, 09:30 PM
So far no leaks. Cleaning product out of glycol w/a pilot unit & daft unit.

botrous
06-09-2005, 09:41 PM
As so far there are no leaks , keep your eyes on the pressures on both sides (as i understood , that the glycol side pressure can't be monitored :( ) but if you can get your boss to agree to install a gauge that will be great :cool:
So keep your eyes and be sure not to get a milkoglycol product , we don't know it's direct or side effects on the public health yet :eek: :eek: :eek: