Ammonia refrigeration “diffuser tanks” have been around since the early 1990’s when the Uniform Fire Code (UFC) began requiring them. That means some of these tanks will be due for their Mechanical Integrity Inspection and this is when most facilities will realize that the process design does not lend itself for a safe means to enter this tank for an internal inspection. And yes, an internal inspection is REQUIRED on these atmospheric tanks when the facility is using the holy grail of atmospheric tank inspection RAGAGEPS… API 653, Tank Inspection, Repair, Alteration, and Reconstruction. Of course, there are other options for conducting atmospheric tank inspections, but those too will require some internal inspection on some fixed frequency. So what does this mean?
API 653 states…
6.4 Internal Inspection 6.4.1 General 6.4.1.1 Internal inspection is primarily required to do as follows.
a) Ensure that the bottom is not severely corroded and leaking.
b) Gather the data necessary for the minimum bottom and shell thickness assessments detailed in Section 6. As applicable, these data shall also take into account external ultrasonic thickness measurements made during in-service inspections (see 6.3.3). c) Identify and evaluate any tank bottom settlement.
6.4.1.2 All tanks shall have a formal internal inspection conducted at the intervals defined by 6.4.2. The authorized inspector shall supervise or conduct a visual examination and assure the quality and completeness of the nondestructive examination (NDE) results. If the internal inspection is required solely for the purpose of determining the condition and integrity of the tank bottom, the internal inspection may be accomplished with the tank in-service utilizing various ultrasonic robotic thickness measurement and other on-stream inspection methods capable of assessing the thickness of the tank bottom, in combination with methods capable of assessing tank bottom integrity as described in 4.4.1. Electromagnetic methods may be used to supplement the on-stream ultrasonic inspection. If an in-service inspection is selected, the data and information collected shall be sufficient to evaluate the thickness, corrosion rate, and integrity of the tank bottom and establish the internal inspection interval, based on tank bottom thickness, corrosion rate, and integrity, utilizing the methods included in this standard.
6.4.2 Inspection Intervals Initial and subsequent inspection intervals shall be in compliance with the requirements of 6.4.2.1 and 6.4.2.2. For existing tanks, tank owner/operators shall review the internal inspection interval and be in compliance with this section within five years from the date of first publication of API Standard 653, Fourth Edition, Addendum 2, January 2012.
6.4.2.1 Initial Internal Inspection Interval The initial internal inspection intervals for newly constructed tanks and/or refurbished tanks shall be established either per 6.4.2.1.1 or 6.4.2.1.2. 6.4.2.1.1 The interval from initial service date until the first internal inspection shall not exceed ten years unless a tank has one or more of the leak prevention, detection, corrosion mitigation, or containment safeguards listed in Table 6.1. The initial internal inspection date shall be based on incremental credits for the additional safeguards in Table 6.1 which are cumulative.
So as we can see, the internal inspection frequency for the vast majority of diffuser tanks will be ten years. The practice does allow for alternative inspection means to be used as well as longer inspection frequencies under specific use and conditions, but the vast majority of internal inspections I have seen done were done via entering these tanks every 10-years. And it is this “entry” I want to discuss here…
Most of my experience is that facilities have not thought about MI inspections for their diffuser tanks and in fact, some of these tanks were constructed absence of any manway large enough to allow entry into the tank. But these tanks, all of the ones I have seen/worked with, were steel tanks and corrosion is a concern and internal inspection is necessary. When the NH3 process is a PSM/RMP covered process, these tanks are without a doubt part of that PSM/RMP covered process as they are interconnected to the covered process AND they are a significant “safety system” within the process. So the argument that the tank is somehow exempt from any inspection following a chosen RAGAGEP is just blatantly false.
The problem lies in how the tank was designed to function, and since this involved an RV discharge there was no consideration for “isolating this tank,” and thus no valves were installed in the line from the RV header to the diffuser tank. And this leads to a serious concern for those entering the tank when the NH3 process is still charged. Even with the process shut-down, there is a chance for an RV to lift, either as needed or prematurely, and this would be funneled directly to the diffuser tank, placing the inspector(s) in extreme danger.
So how does one enter a diffuser tank SAFELY when the NH3 process still contains a charge of NH3?
The first option we should look for is a means to DISCONNECT and MISALIGN the vent pipe to the tank. Knowing that NH3 pipe is welded, this means that we would have to have at least one section of the vent pipe to the tank installed using flanged connections. We would want this flanged spool piece to be at the height of 15′ and at least 20′ from any building openings or intakes. One such design was such that the spool piece was at the tank nozzle on top of the tank. The team would disconnect the flanges and rotate the pipe so that the end of the pipe is pointed upward. They would then install a 7′ piece of pipe that would put the RV discharge 7′ above the tank, and they would install a blank on the tank nozzle. This allows for the tank to be PROPERLY ISOLATED from the hazard AND should one of the RV’s lift, as needed or prematurely, the pressure is allowed to discharge freely to the atmosphere.
But what if the pipe is welded all the way to the bottom of the diffuser tank? That poses a significant challenge when the process still contains NH3. Of course, the simple answer (NOT a simple solution) is to remove all the NH3 from the process and place the entire process into a ZERO ENERGY STATE in regards to NH3. But that has significant risks and should not be done if it can be avoided.
I know a lot of businesses that utilize ammonia refrigeration do not like hearing this, but there is a way that we can place an “intervening valve” with a properly sized vent between the process and the diffuser tank. This intervening valve MUST be “car-sealed” OPEN and the vent valve “car-sealed” CLOSED during normal operations. We MUST remember that these valves internals MUST not pose any restriction in our RV Design. But this system also needs to be set up so that the end of the vent line is 15′ above ground and 20′ from any building openings/intakes. When our process has this type of set-up, we MUST have a written “car-seal” program (or some kind of administrative program) that will ENSURE the relief system is NEVER compromised UNTIL we have put the process into a condition where it no longer needs relief protection. This means that these intervening valves MUST be sealed in their SAFE position and they need to be frequently inspected to ensure they remain that way. In order to deviate these valves, it takes a multi-layered approval process, AND then there MUST be a multi-layered procedure to ensure they are placed BACK INTO their SAFE POSITION when the work is done.
Either way, these diffuser tanks are PSM/RMP covered vessels and MUST be included in the MI inspection/testing program, and this inspection/testing program will most likely call for an internal inspection at some frequency. This is in fact why OSHA (and EPA) call out Confined Space Entry as a necessary Safe Work Practice in a PSM/RMP program. How we enter the diffuser tank can be quite a challenge.