Monday, February 12, 2007

Fate Was My Partner

It was a hectic year. The Vietnam buildup was in full swing. We were building major projects on the largest military base in the north east. The project we were working on now consisted of 30 barracks a Visitors officers Quarters and a variety of other supporting facilities including several mess halls, refrigeration buildings and a large power plant.

I had a team of 10, we were responsible to inspect, test and accept buildings and systems as the contractors finished the work. This week, everything was moving like clock work, one might even say we were all moving and interfacing like a professional ballet group. Everyone knew what they had to do and when they had to do it. Most of us were working together as teams all year so we knew each other very well and we knew what to expect from each other.

Marty and his team worked for the contractors, they were good very good. This week we were working on the refrigeration building and finishing up some remaining systems that we could not finish before, because we were waiting for equipment. Marty was the Chief welder, he could weld a 6 inch joint with his eyes closed upside down or on his back...he was very good.

It was the Friday before a three day weekend. We all wanted to leave that day at 2 P.M. First thing in the morning I had my usual safety meeting, I rushed it a little. I always covered safety tips during the meeting geared to what we were going to do that day. Marty and his guys were pros, they heard the speech a hundred times before..but they were always polite. After the meeting we had our usual coffee talked about what we were going to accomplish that day and than started work. As we worked we would joke and kid each other and talk about what we were each going to do that weekend. It was a typical day, nothing out of the ordinary.

We broke for lunch, joked, threw a ball around and everyone went back to work as if we were in some kind of a trance...it was just automatic. We were moving at a good clip. It was obvious that leaving at 2 P.M. was not going to be a problem. It was about 12:30, Marty and his team were way ahead of my team and we were trying to catch up. I bent down to pick up a water bottle, Ron on my team, who was standing about two feet from me on my right side was installing a test gage. Sidney on our team was just about a foot next to Ron. At that moment we were all lined up, as if we formed a valley and we three were the sides.

Marty and his guys just finished all the welding for the 6 inch connection for a five foot long, one foot diameter steel exhaust muffler for one of the large compressors, it weighed about 250 pounds. The electricians turned the power on, the compressor struggled at first, coughed and at the instant I bent down to get my bottle of water there was a loud explosion. I spun around, Ron threw his hands up to his ears and Sid just turned to look.

I froze as the 250# steel projectile came flying by me missing my backside by about an inch, Sid hit the deck and Ron moved his arm but not enough. The 250# projectile flew by him but managed to skim his arm, enough to give him a mean third degree burn and break his elbow.

The entire event must have taken just 2 or three minutes but to all of us it seemed like an eternity. I rushed to the emergency base phone and called emergency services. With them came a base safety inspector. We spent that three day weekend going over everything that happened piece by piece to see if we could figure out what happened. Marty's welding and silver soldering was torn apart and inspected over and over again. Nothing was found. The inspectors sent the joints out to a lab to be examined 100 different ways...nothing was ever found.

Fate watched over all of us that day. When we went back to work the following week, it was never the same. Why did it happen? How is it none of us were killed? What made that 250# projectile take the path that missed all of our vital parts?

We never found out, and we knew we would never know. But we all learned that day that once fate is the hunter....the event is taken out of our hands.

Hal

Saturday, February 10, 2007

Effects of Two Phase Steam Flow

Again, it is important to realize that the theoretical and working flow equations for all flow controls and meters are based on homogeneous flows. Depending on the flow controlling devices principal of operation, the effect of non-homogeneous flow can be considerable. Two phase fluids such as wet steam, play havoc with the accuracy of steam flow measuring devices and steam flow controls. For example. With a device that utilized an orifice to measure and or control steam flow, operating at a constant 150 psig going from saturated steam to about 10% moisture could cause data supplied to the control device to be considerably lower than actual flow, depending on how the moisture isdisbursedd within the steam.

Two significant differences with non-homogeneous flows are that first, the density is not easily derived or measured, and second, one phase of one or more of the fluids components may not be moving at the same velocity as the main flow. Therefore, in these cases some of the fluid may actually be flowing along the bottom of the pipe and be separated out, alternating between a separated flow and an entrained flow in different parts of the distribution system. When the liquid droplets separate out of the flow, the phenomena is commonly called slip. Slip is a complex function of viscosity, particle size, density differences, surface temperature and the superficial velocity of each component. The effects of gravity also play a part in slip and a part in altering various flow patterns. Horizontal, vertical and inclined pipes cause differing relative velocities for the varying components, so they each affect slip differently.

Whenever you have a two phase flow which must be controlled and metered, the void fraction, or percentage of each constitute, must be determined in order to predict the quantity or velocity of each component. Therefore, measuring such a two phase flow with a simple orifice designed under homogeneous equations and equations which apply to saturated and single phase flow, will contribute considerable errors in actual field measurements. Additionally, if the steam pressure supplied to a particular device is above the original design pressure of the measuring device and that device does not have pressure compensation, the combination of wet steam and increased steam pressure will generate a substantially reduced reading from any meter or flow control. For example. At 10% moisture content and 160 psig steam in lieu of 125 psig as the original measuring device may have called for, the differential pressure metering device will register only 85% of what the actual flow is.

One way to protect against meter and flow control inaccuracies created by a two phase steam flow is to utilize moisture separators upstream from the measuring device. If such separators have not been utilized and there are indications that excessive moisture exists within the distribution system, one should immediately suspect that meter readings, flow indications and flow control, are all being adversely affected.

If previously installed moisture separators are constantly overloading and damage to existing orifice plates, PRV stations and meters appear to be caused by excessive condensate, immediate action should be taken to determine the cause of the increase in the wetness of the steam.

Moisture in the steam flow that amounts to over 2% wetness has a tendency to also act as a grinding agent on orifice plates and other types of differential pressure measuring devices, causing enlargements in the measuring plate or device. The coefficients utilized therefore in the various equations and algorithms, though correct at the time they were selected since they were related to the design orifice diameters, will be incorrect when applied against the enlarged orifices which have enlarged as a result of the grinding effect caused by the wetness of the steam. The excessive moisture in the steam will also cause orifice plates to become warped. The excessive moisture, even if it is not in the form of large slugs, will have an eroding effect on orifices, control valves and on the PRV valves located as part of pressure reducing stations. Allowing this to happen can cause serious errors and dangers within a steam distribution system.

For more information on this subject please see the papers I have written and the book I wrote, Steam Distribution and Flow: A Guide for High, Low and Medium Pressure Systems, available at
http://www.nrctraining.com/.

Hal