by Rabbi Simcha Smolensky, Senior RFR
I’ve thought many times about the fluid dynamic physics of product (i.e., oil, chocolate, anything liquid) as well as water (as is used for kashering) moving through pipes. We typically kasher pipes by pumping boiling water through them. Thinking about the physics of this movement, there are a few questions that come up with this kashering procedure:
- Does the water retain kashering temperature through to the end of the pipe run? This is obviously a bigger question with very long lines and more so with lines exposed to cooler external temperatures (like un-insulated pipes outside).
- Does kashering water actually hit all the interior surfaces of the pipe as it is going through?
The first question is simple to solve – checking the temperature at the exit of the pipe run will easily show if sufficient temperature has been maintained for the distance. Adjusting the quantity of water to sufficiently heat, and maintain temperature, in the pipe may be necessary.
The second question is more interesting from a physics standpoint. Consider the simple system pictured here. Tank A is the supply. Liquid comes out the bottom of the tank, runs through a pump which raises the liquid up to a transfer line which in turn empties into Tank B. Assuming the liquid regularly running though this system is hot, we would require kashering the tanks and lines if they became non-kosher or dairy to return them, respectively, to kosher or pareve status. Kashering would typically be with hot water running the same route as product took – A through the pump, up and over to B.
We all know that kashering is accomplished by all surfaces of a kli coming into contact with boiling water, not necessarily simultaneously but eventually. My question really boils down to… does boiling water ever contact the top of the pipe running horizontally and then down into B (the red section of the diagram)?
If we think about this logically, the pump is going to be pushing liquid up the vertical riser pipe and that section of pipe is certainly going to be filled with liquid since gravity is pulling the liquid down and the pump is being used to overcome the force of gravity. But once liquid reaches the top, gravity acts to exert force on the water to stay at the bottom, or lowest point, of the cross (horizontal) part of the line. As water reaches the top of the riser, the pulses of liquid being pushed out by the pump should flow by the path of least resistance, dissipating across the horizontal pipe and then dropping down into tank B (assuming there are no blockages or restrictions to the pipe diameter). As soon as the first liquid reaches the top of the riser it will encounter no further gravitational resistance and be able to flow much faster along the horizontal pipe. That would mean that the interior top of the cross member pipe would not be touched by kashering water!
At the 2014 IFT show, I sought out the engineers from the Murzan Company. Murzan specializes in designing piping and pump systems for food plants. I discussed my thoughts with them, and they confirmed that my suspicions are correct – with the cross pipe in the diagram being level and not restricted in any way, it will never be filled with liquid unless the viscosity of the liquid being pumped is extremely high and therefore not free flowing. In the case of water, oil or similar liquids the horizontal pipe will never be filled. They said that it might be possible to oversize a pump to force more liquid through at a very high flow rate, but they thought that even then the runoff across the span of the horizontal pipe would still result in that pipe never being filled. The larger the diameter of the pipe, the more profound this effect would likely be.
It goes without saying that if the cross pipe has a downward slope at all, the effect would be even more magnified and even less depth of water would be observed in the pipe.
If the pipe was not hung level, but the end was at least one pipe diameter higher than the origin, the pipe would indeed fill. Or, if there was a restriction on the end of the pipe such as a partially open valve or a reduction of diameter (from larger pipe to smaller) the cross member could also fill since there would be back resistance and the pump may be able to push more liquid into the cross pipe than is free-flowing out.
I asked the engineers if there was a way to detect this effect, for example if a magnetic thermometer was coupled to the very top of the pipe and another to the bottom, could we see a temperature differential that would indicate the top of the pipe was not in contact with liquid inside? They said that the metal of the pipe is too conductive and there would be no discernible difference in the temperatures.
This last point may have a bearing on the question in terms of halacha – if liquid doesn’t fill the pipe, perhaps we could say that our kashering which also doesn’t fill the pipe is adequate since the kashering water contacts the same place that product does. This may be problematic in a couple of ways: 1) if the pipe is conductive enough with heat, would we say the blios of NK or dairy in effect travel and permeate all the metal of the pipe? And 2) Perhaps water is not as viscous as the material that flowed through the pipe that caused the pipe to be dairy or non-kosher, and therefore the water doesn’t even reach all the parts of the pipe that product did contact?
If in fact this is a valid concern, there is really one simple solution – when kashering such pipes with hot water, the end of the line must have some flow restriction placed on it to make sure the transfer pipe fills completely with water. Mashgichim who are involved in kashering pipes need to assess the specific situation of the pipes that are to be koshered, identifying where this effect presents an issue, and adjusting the procedure during kashering accordingly. If the mashgiach is not comfortable understanding the exact nature of the piping being koshered, he should seek assistance from senior field staff or office staff resources.
Rabbi Gersten reports that Rav Schachter read this article and asked that this important issue be publicized. He also had two points that are worth noting:
- If all areas of the pipe that had contact with product, will have contact with boiling water, then bidieved the pipe is kashered. Although lichatchila one should kasher the entire kli, even areas that did not have contact with product, but when this is not possible, it is not required (see Shach Y.D. 121:18).
- If it is not possible to restrict the flow, and build up pressure in the line, one other possibility for kashering would be to run the kashering water through the pipe for an extended length of time (e.g. an additional 10-15 minutes). By doing so, although the top of the pipe will not fill with water, it will fill with steam. The condensation that forms on the top of the pipe from the steam can be used to kasher the pipe (see Igeros Moshe Y.D. I:60).