HVAC Chilled Water System

so?

The second part of that post contradicts the first.

Go ahead.

The odds are in your favor.

<Exit, stage left>

" It is important to note that the direction of flow must confirm with the direction of the arrow on the valve body and that the valve must be installed with a minimum of 3 D (3 x nominal pipe diameter) straight pipe in the upstream side. "

http://www.flow-techs.com/Double%20Regulating%20Valves.pdf

The arrow (if any) on a valve does not always refer to the direction of normal flow through the valve. Instead, it may refer to the direction in which the valve can stop flow. Those are not the same thing, but the variations can be complicated to explain, and even would-be "valve engineers" sometimes get them wrong.

I'm in no way expert on valves, and I'm certainly NOT saying you are wrong, but...

1. Wouldn't the normal flow direction be the same as the direction it can stop flow for ordinary valves?

2. Wouldn't the normal flow direction be the opposite from the direction it can stop flow for anti-backflow valves?

eg.

Unless I'm mistaken, normal flow for the valve shown is to the left, as indicated by the arrow, and flow is prevented in the opposite direction.

Any comments, corrections, or clarifications?

That is a check valve, which is different from an ordinary valve.

In refrigerant control and some other valves, the arrow does NOT necessarily mean what you think it means. The specific variations can be lengthy to describe, which is why I didn't try, but they depend on which side of the valve is under the greater pressure when the valve is meant to be closed, and also to special characteristics of solenoid valves, regulators, etc.

If you had to enlarge the intentionally selected under-sized valve after installation to meet system flow demand requirements, then there is not any economical advantage what-so-ever gained by purchasing and installing the under-sized valve.

In fact quite the contrary is true in that the system now has a custom, one-of-a-kind modified valve that will cause severe costly maintenance issues and poor flow control for the customer.

Just so you understand:

Operating the 6 inch valve in the 70-80% range ensures the most accurate linear flow control possible is attained which translates to accurate temperature control, lower maintenance costs, and higher system reliability.

Operating a 5 inch valve in the 90-100% range ensures the flow will be erratic and not linear at all causing system control to be unreliable and increasing maintenance costs.

Initial cost should be the last option to consider when designing and installing any HVAC system as the added energy and maintenance costs build up very quickly and often cause premature failure of the system components and poor system operation.

This is a typical example of a very poor decision based upon false budgetary savings.

Look into the Cv of the valve. It has to do with the flow characteristics of the valve.

I often explain it this way. When you turn the water to a garden hose, you can crack the valve open and get a little flow. Turn it a quarter turn and you get about 50% of the full flow. Going another quarter turn you get about 80% of full flow. Then you can turn the knob 3 more times and the flow doesn't really change at all. - This valve have poor flow chararcteristics for controllability.

When a valve is too big you'll get 80% flow when the valve in 30% open, so you end up trying to controll everything between 0-30% open. This causes the valve to wear out fast with something called 'wire drawing' effect.

If you put a 6" valve in a 6" line, it will likely be difficult to control. You may need a 5" or 4" valve. If you want to just guess and not deal with Cv you can just go down 1 size.

http://www.engineeringtoolbox.com/valve-selection-guide-d_35.html

As a follow up to my earlier reply the above link will supply extremely valuable information on proper control valve selection for achieving linear flow characteristics.

There are many other such documents available on the web for free downloading from other reliable, competent sources.

Most valve manufacturers offer cage, seat, shaft, and "spintle" replacement options that include a new valve information tag illustrating the modifications so as to avoid future issues caused by undocumented valve modification.