In the previous
article, I introduced you to the elements of which the ball valve folds. Today
I'll show you what are the alternative stem seal assemblies.
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| Cross-section of the ball valve |
The stem design is
responsible for the reliability of the pivoting mechanism handles. These blocks
can be of two types: maintainable or not.
When using the device,
there is a possibility of leakage of water from under the handle. This is due
to non-compliance
Conditions of use or
when exceeding the maximum permissible operating parameters (temperature,
pressure). A ball valve with a repairable stem assembly allows you to extend
the product life cycle. This is achieved by tightening the stuffing box.
If a leak occurs in the
device with a non-reusable rod assembly, it must be dismantled and replaced
with a new one. And this is a waste of time and money. Variants 4-8 are nodes
with the possibility of repair.
In this article I
identified 8 basic options:
1. With two NBR
elastomer rings
2. With two rings of
elastomers NBR and FPM
3. With two rings of
elastomers NBR and FPM and Teflon gasket
4. With Teflon gasket
and stem inserted from the inside
5. With teflon gasket
and NBR ring. Clamping nut with internal thread
6. With teflon gasket
and NBR ring, and a conventional nut
7. With two Teflon
gaskets and NBR ring. Clamping nut with internal thread
8. With Teflon gasket
and stem inserted from the outside
I want to focus your
attention on such details.
Way of insertion of the stem.
It can be inserted
either from the outside of the valve body (8) or from the inside (1-7). The
first option is unsafe. When the handle of the valve is removed (for
sanitation, for example), there is a possibility that the stem can fly out of
the casing. This is due to the pressure of water. Another modification includes
a special shoulder. It also protects
the stem from the outward flight.
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| Maintainable stem assembly |
Thread of the clamping nut.
It happens, both with
an internal carving, and with external. The design with an external thread is
more reliable. Since the internal thread loosens the rod.
Seal materials.
Seals are made of
various materials:
- Elastomer NBR;
- FPM elastomer;
- Teflon PTFE.
The best performance
and technical features have the latest material. Thus, the first two materials
do not have such characteristics of high temperature resistance as Teflon.
The latter has a high
degree of slip (small coefficient of friction). This increases his "life span".
Repairability.
This fact was considered above. |
| Stem of ball valve |
Design.
What is the best
design?
In my opinion, this is an option number 4. |
| PTFE seal |
Since it has the
following advantages:
- Safety (the stem is
inserted from the inside);
- Quality (due to the
use of Teflon);
- Reparability (design
with a clamping nut);
- Durability (Teflon is
selected as the seal);
- Reliability (the
clamping nut includes external thread);
- Availability (the
price of reinforcement with such a configuration of the stock node is in the
price / quality range).
Some shut-in developers
offer different options. For example, with two Teflon conical gaskets (7).
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| Stem with two Teflon conical gaskets |
This configuration
provides, as it were, automatic control of the degree of compaction from the
side of increased pressure. But this is not yet proved in real conditions and
affects the price.
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