Canned Pumps and Process Connections

[sirdle]

I just hate it when I discover that I may have been doing something wrong for years...

Article 501.5(F)(3) discusses drain and sealing requirements for Canned Pumps, Process or Service Connections.

1. What is a canned pump?

2. By process or service connections are we talking about all those electrical switches and transmitters that connect to, and measure, the process conditions in a plant? All the temperature, level, flow, and pressure devices?

Let's say we have an explosionproof pressure switch that connects to a vessel and measures pressure at 600 psig.

Inside the explosionproof junction box is a bourdon tube that is connected through mechanical linkage to a microswitch. The bourdon tube provides a primary seal to isolate the process pressure from the electrical system. The electrical conduit connected to this explosionproof junction box is sealed to maintain the explosionproof integrity of the box, but it is not designed to withstand a continuous pressure across the seal. So if the primary seal fails, the process fluid could enter the electrical conduit.

In this case an additional "approved" seal is required and a means of draining the fluid needs to be provided as a visual indication that the primary seal has failed. Am I reading this correctly? API RP 14F takes this a step further and requires that the drain be capable of reducing the "fluid pressure under dynamic conditions to less than 6 inches of water column upon failure of primary seal." ... That would be some drain!

3. I have never seen a field device (switch or transmitter) labeled with the number of seals it has internally. Would this be part of the listing requirements? And if it is not listed or labeled how can I determine if it meets the requirement of Article 501.5(F)(3)?

4. What is this additional "approved seal, barrier, or other means" which can be used? I have never seen one installed and I have never seen one in any trade literature.

Or am I reading this whole section wrong?

Any help would be appreciated.

 

[rbalex]

Re: Canned Pumps and Process Connections

1. There are several forms, but basically what they all have in common is the process fluid is used to cool (and possibly lubricate) the driver by passing through it as well as the pump. Since there is no oxygen in the flow, there is no direct ignition method. Nevertheless, the power conductors must somehow get to the stator windings, typically through a sealing method consistent with the pressure/temperature of the process at the point of the pump.

2. Yes, this is basically correct. The other devices in the ?list? ?canned pumps, process, or service connections for flow, pressure, or analysis measurement, and so forth?? are instrumentation equipment directly connected to the process equipment. You are also correct that ?typical? explosion-proof seals are not designed for continuous service under direct process parameters and a failure of the primary instrument ??seal, barrier or other means?? must have a relief method between it and the first explosion-proof seal.

3. Most manufacturers of this class of equipment make ?double-sealed? devices with a mechanical seal between the instrument and the ?process? and another between the instrument and the electrical connection enclosure. Both are typically full process rated and there is typically a process relief method between them. Check with the manufacture - not the sales rep. It is not necessarily part of the listing but it is readily a specifiable feature.

4. If it is a field installed seal, there are typically two standard explosion-proof seals with a drain method between them. In this case it is essential to terminate cables and conductors per [2005] 501.15(D) or 501.15(E) as appropriate. (You should anyway of course, but it even more important here)

[ July 13, 2005, 08:59 PM: Message edited by: rbalex ]