Stainless steel ASME pressure vessels are very expensive and long-lived pieces of process equipment. Many tanks in use today still have years of good service left in them, but processes keep getting cleaner year by year.
A tank can easily become unusable when a new aseptic process comes along. The surplus of these once-useful tanks, and the prices and long lead times of new tanks (now as much as 16 weeks at some plants), accounts for the increasing number of tank retrofits being carried out across the country.
How can these retrofits be done successfully?
When a tank is retrofitted, components that present a risk of contaminating the new product are replaced.
Old tank outlet valves are a critical replacement item, and ASEPCO valves are a popular choice as replacements. ASEPCO valves are commonly chosen to replace ball valves, rising stem valves with O-ring seals, and weir-type diaphragm valves to improve cleanability and simpler, less-costly maintenance.
What’s the best approach? Should I recode the vessel or buy a custom retrofit valve body designed to fit the current tank flange?
There are two basic approaches to a tank valve retrofit.
The first approach is to cut the old valve out of the tank and weld in a new one. If the tank is a pressure vessel, it must be hydrotested in the presence of an ASME pressure vessel inspector. This process brings it into compliance with the pressure vessel code. It has been re-coded.
The second approach avoids cutting anything out of the tank. Instead you would leave the old coded flange welded to the tank, unbolt the original valve, and bolt on a new aseptic tank valve whose inlet is specially designed to fit the flange already on the tank. This is called a custom retrofit and avoids the need to recode the pressure vessel.
Most engineers we’ve talked to assume that recoding an ASME pressure vessel is a complexity just shy of writing a Ph.D. dissertation on black holes. This isn’t the case. When you use a reputable ASME tank maker to handle the recode in order to get top notch, reliable results—sometimes quicker than you’d think.
Some tank companies only offer recoding at their facilities, which means taking the tank out of service for a long period. Others offer on-site recoding—with some able to work on amazing 24-hour turnarounds.
The recode is more expensive than having a custom valve made to fit the flange already on the vessel. We’re familiar with this process because we’ve provided lots of valves for both methods. And the recode method is clearly the dependable, long-term answer. Here’s why.
Tank outlet valves are upgraded for two reasons:
- To increase conditions of asepsis by using an aseptic valve designed to eliminate contamination issues.
- To offer a flush valve seat, which ensures an absence of a dead zone. A flush valve seat stops product from dropping into a pocket or dead zone.
Dead zones create three kinds of problems:
- Uneven product concentrations, which cause filling problem
- Low O2 concentration in the dead zone causing cell death
- Pockets of solids that settle into the dead zone and interfere with tank draining
While a bolt-on retrofit valve is perhaps cheaper, it can never seat flush to the tank bottom. Drawings 1, 2, and 3 show why.
To the right, we show three ASEPCO retrofit designs. Two of these options are bolt-on retrofit designs that must attach to the flange already on the vessel. This creates some dead zone with any flush-mounted diaphragm valve. Compare the ASEPCO flush mount with the bolt-on configurations in Drawings #1 and #2.
As you can see, the custom retrofit valves have a large dead zone. This is inevitable even though our engineers design to minimize this dead zone. The reason is that the custom retrofit can only be mounted to the bottom of the flange, which isn’t flush to the tank bottom to begin with.
So, while the custom bolt-on ASEPCO valve is cheaper than a full cut-out-replace-recode job, it is certainly not better, unless the application is indifferent to the dead zone.
There are three critical questions to ask as you decide whether to cut out the old valve flange and replace it with an ASEPCO Tank-Bottom Valve, or to bolt-on a custom retrofit to the existing flange:
- What level of asepsis and reduction of dead zones must be reached now?
- Will future products be able to tolerate a dead zone if we go the bolt-on route?
- How soon will this tank need another upgrade to remove the dead zone?
It’s smart to consider Question 3 carefully. Our industry evolves quickly and processes will likely change again in the tank’s lifetime, perhaps more than once. The trend is always toward cleaner processes. The analytical tools to detect dirt advance very quickly, and the FDA’s interested in requiring cleaner processes when such cleanliness protects the customer.
If the vessel cannot tolerate a dead zone at all, then the removal of the old tank flange and addition of an ASEPCO Tank-Bottom Valve, with associated recode, is the best solution.
On the other hand, if a dead zone is tolerable then selecting a custom retrofit can be the right answer. But you must ask yourself whether you can see a general trend toward aseptic products at your company.
If so, the old tank may need an aseptic valve without a dead zone in the near future. Then it can be much wiser to put on a new ASEPCO Tank-Bottom Valve now.
Engineers tell us that it is far better to be remembered as the one who committed to the recode process slightly before its time than to be remembered as the engineer who ordered a costly custom valve, only to find that its dead leg soon proved unacceptable.
By the way, ASEPCO has built custom bolt on retrofit valves for tanks that—within 12 months—were recoded and fitted with our standard ASEPCO Tank-Bottom Valve.
Thus, the custom retrofit valve is a quicker and less expensive solution to the tank upgrade requirement.
But it may be temporary.
Mounting a new ASEPCO valve and recoding the tank would be the more costly, but permanent solution.
Regardless, if you specify an ASEPCO valve, you can know that you are using the most reliable aseptic valve technology available. You’re also using the lowest long-term cost diaphragm valve in our industry.
Remember, ASEPCO valves save you 80% of the maintenance costs of weir type diaphragm valves—for the life of the valve. No more torque wrenches. No more 15 minutes-per-valve diaphragm changes. Instead, your maintenance techs can use a simple tri-clamp and 1 minute of work to get the most rugged and reliable diaphragm seal in aseptic processing.