压力表(中英)

压力表

Introduction to Pressure Gauges

压力表

和压力开关

< br>压力表和压力开关是工厂中最常用的仪器。然而，由于为数众多，维护和可靠

性经 常会被忽视。从而，在老旧厂区见到大量无法使用的压力表和压力开关就

不足为奇。这种 情况非常危险，因为如果压力开关失灵，工厂的安全就得不到

保障。反之，如果压力表损 坏时工厂依然能安全运转，则说明该仪表毫无使用

的必要。因此，良好的过程仪表设计的 首要目标便是，以较少的压力表和压力

开关获得较好的作用和可靠性。

< br>

减少工厂中仪表数量的一种有效方法是，

（例如

在每个泵的出 口都安装一块压力表）

。而是逐个核对每个设备的需要。在核对

过程中，我们应该思考以下问题：

“

这个压力表的读数对我有什 么用处？

”

，只

有在得到合理的答案时 ，才需要安装压力表。如果压力表只是用来指示泵是否

正在运转，就没必要安装，因为无 需仪器即可做出判断。如果压力表指示的是

过程控制中的压力（或压降）

，则只有当我们能对结果采取相应措施（例如清

洁滤清器）时这些信息才有意义 ，否则就毫无用处。如果以上述原则来使用压

力表，所用的压力表数量就会大幅减少。如 果工厂使用的压力表少而精，可靠

性也会随之升高。

Pressure Gauges and switches

Pressure gauges and switches are among the most often used instruments in a plant. But

because of their great numbers, attention to maintenance--and reliability--can be

compromised. As a consequence, it is not uncommon in older plants to see many gauges and

switches out of service. This is unfortunate because, if a plant is operated with a failed pressure

switch, the safety of the plant may be compromised. Conversely, if a plant can operate safely

while a gauge is defective, it shows that the gauge was not needed in the first place. Therefore,

one goal of good process instrumentation design is to install fewer but more useful and more

reliable pressure gauges and switches.

One way to reduce the number of gauges in a plant is to stop installing them on the basis of

habit (such as placing a pressure gauge on the discharge of every pump). Instead, review the

need for each device individually. During the review one should ask:

reading of this gauge?

only indicates that a pump is running, it is not needed, since one can hear and see that. If the

gauge indicates the pressure (or pressure drop) in the process, that information is valuable only

if one can do something about it (like cleaning a filter); otherwise it is useless. If one approaches

the specification of pressure gauges with this mentality, the number of gauges used will be

reduced. If a plant uses fewer, better gauges, reliability will increase.

压力表

设计

压力表（和开关）损坏的两个常见原因是管道振动和水气凝结，在寒冷天气

下会冻结和损坏仪表外壳。图

1

所示为传统压力表和更可靠的

“

充液式

”

压力

表。传统压力表的专用连杆、枢轴和小齿轮对水 气凝结和振动都十分敏感。充

液式压力表的使用寿命会更长，不只是因为它的移动部件较 少，还因为它的外

壳充满了粘稠的机油。充油带来的好处不仅有指针减振，还可以阻止周 围环境

中的潮湿空气进入仪表。如此一来，水气就无法凝结和积聚。仪表的特性还包

括可以提高可见度的表盘照明和数字读数，

能够纠正环境温度变化误 差的温度

补偿功能，用于测量压差的压差表以及在同一表盘指示两个压力读数的双针

表。

压力表根据精度可以分为

4A

级

（允许误差为满量程的

0.1%

）

到

D

级

（误

差为

5%

）

。

Pressure Gauge Designs

Two common reasons for gauge (and switch) failure are pipe vibration and water

condensation, which in colder climates can freeze and damage the gauge housing. Figure 1

illustrates the design of both a traditional and a more reliable,

delicate links, pivots, and pinions of a traditional gauge are sensitive to both condensation and

vibration. The life of the filled gauge is longer

, not only because it has fewer moving parts, but

because its housing is filled with a viscous oil. This oil filling is beneficial not only because it

dampens pointer vibration, but also because it leaves no room for humid ambient air to enter

. As

a result, water cannot condense and accumulate. Available gauge features include illuminated

dials and digital readouts for better visibility, temperature compensation to correct for ambient

temperature variation, differential gauges for differential pressures, and duplex gauges for dual

pressure indication on the same dial. Pressure gauges are classified according to their precision,

from grade 4A (permissible error of 0.1% of span) to grade D (5% error).

防护配件

Protective Accessories

压力表最主要的配件是仪表与工业过程之间的截流阀（图

5-2

）

，截流阀在仪

表拆卸或维护时起到拦截作用。此外经常还会出于以下两个原因而加装辅助

阀：在蒸 汽（例如水蒸气）中使用时排出冷凝水，或是在高精度应用中对外部

压力源进行校准。< /p>

其它配件还包括盘管或虹吸管（图

5 -2A

），它们可以保护仪表在蒸

汽中使用时免于高温损坏；以 及缓冲器或脉动缓冲器（图

5-2B

），

它们可以吸收压力冲击并平衡压力波动。

如果需要冻结保护，

应使用

蒸汽或电伴热对仪表进行加热。化学密封（图

5-2 C

）可避免仪表在

粘稠浆液中使用时发生堵塞，

并防止腐蚀性、

有害或有毒物质接触传

感器。它们可以 防止过程液体在传感器末端空腔中发生冻结或胶凝。

密封通过工业过程和仪表之间的一层 隔膜来保护仪表。

仪表和隔膜之

间的空腔中充满了稳定、热膨胀 系数低、粘性低且无腐蚀性的液体。

在高温应用中通常使用钠钾共晶合金，

在室温下使用甘油和水的混合

液，在低温下则使用乙醇、甲苯或硅油。

如果化学密封通过毛细管与压力表相连，

便可将压力表放置到操作员

视野更好的位置。

为了保持精度，< /p>

毛细管不能暴露在过高温度下，

长

度也不 能超过

25

英尺（

7.5

米）。有四种化学密封方式：离线式、

“流通”自洁式、密封件探入式 或法兰连接式。

化学密封中隔膜的弹簧刚度可能导致在低压

（低于

50

psig

，

350

< br>kPa

）

和真空环境

（溶解在填 充液体中的气泡可能从溶液中析出）

下检测时

出现测量错误。< /p>

因此，

这类应用中的密封应优先选用压力中继器。

我

们提供的压力中继器的量程精度范围为

0.1%

到

1%

，绝对压力范围

为

0-5 mm Hg

到

0-50 psia

（

0-0.7

到

0-350 kPa

）。

Protective Accessories

The most obvious gauge accessory is a shutoff valve between it and the process (Figure 5-2),

which allows blocking while removing or performing maintenance. A second valve is often added

for one of two reasons: draining of condensate in vapor service (such as steam), or, for higher

accuracy applications, to allow calibration against an external pressure source.

Other accessories include pipe coils or siphons (Figure 5-2A), which in steam service protect

the gauge from temperature damage, and snubbers or pulsation dampeners (Figure 5-2B),

which can both absorb pressure shocks and average out pressure fluctuations. If freeze

protection is needed, the gauge should be heated by steam or electric tracing. Chemical seals

(Figure 5-2C) protect the gauge from plugging up in viscous or slurry service, and prevent

corrosive, noxious or poisonous process materials from reaching the sensor. They also keep the

process fluid from freezing or gelling in a dead-ended sensor cavity. The seal protects the gauge

by placing a diaphragm between the process and the gauge. The cavity between the gauge and

the diaphragm is filled with a stable, low thermal expansion, low viscosity and non-corrosive

fluid. For high temperature applications, a sodium-potassium eutectic often is used; at ambient

temperatures, a mixture of glycerine and water; and at low temperatures, ethyl alcohol,

toluene, or silicon oil.

The pressure gauge can be located for better operator visibility if the chemical seal is

connected to the gauge by a capillary tube. To maintain accuracy, capillary tubes should not be

exposed to excessive temperatures and should not exceed 25 feet (7.5 m) in length. The

chemical seal itself can be of four designs: off line,

seal elements, or wafer elements that fit between flanges.

The spring rate of the diaphragm in the chemical seal can cause measurement errors when

detecting low pressures (under 50 psig, 350 kPa) and in vacuum service (because gas bubbles

dissolved in the filling fluid might come out of solution). For these reasons, pressure repeaters

often are preferred to seals in such service. Pressure repeaters are available with 0.1% to 1%

of span accuracy and with absolute pressure ranges from 0-5 mm Hg to 0-50 psia (0-0.7 to

0-350 kPa).