دستگاه های کاهش فشار (PRD) به طور گسترده و مؤثری برای محافظت از تجهیزات فرآیندی مانند سیستم های لوله کشی، مخازن تحت فشار، ستون های تقطیر و سایر تجهیزات در برابر فشارهایی که بیش از میزان فشار طراحی شده بیش از مقدار از پیش تعیین شده ثابت هستند، استفاده می شوند. هدف از شیرهای فشار شکن جلوگیری از آسیب به تجهیزات، جلوگیری از آسیب به پرسنل و جلوگیری از خطرات احتمالی برای محیط زیست است.
حداکثر طول خط دریچه تخلیه
The equation for the maximum length of a relief vent line is:
پ1 = 0.25 × [(تنظیم فشار × 1.1) + 14.7]
پ2 = [(تنظیم فشار × 1.1) + 14.7]
L = حداکثر طول خط دریچه کمکی (پا)
D = قطر داخلی لوله (اینچ)
C = حداقل تخلیه هوا (پوند/دقیقه)
The first term in the equation, \(\frac{9 \times P_1^2 \times D^5}{C^2}\), represents the pressure drop in the relief vent line due to friction. The second term in the equation, \(\frac{9 \times P_2^2 \times D^5}{16 \times C^2}\), represents the pressure drop in the relief vent line due to the expansion of the gas as it flows through the pipe.
The equation is set equal to zero because it represents the maximum length of the relief vent line for which the pressure drop will not exceed the set pressure of the relief valve. If the length of the relief vent line is greater than the maximum length, then the pressure drop in the line will exceed the set pressure of the relief valve, and the valve will not open properly.
The equation can be used to design relief vent lines for a variety of applications, such as pressure vessels, boilers, and compressors. It is important to note that the equation is only valid for single-phase gas flow. If the fluid flowing through the relief vent line is a two-phase mixture of gas and liquid, then the equation will need to be modified.
Here is an example of how to use the equation to calculate the maximum length of a relief vent line:
The first step is to calculate the back pressure at the relief valve outlet:
P_1 = 0.25 * [(150 psig + 14.7 psia) * 1.1] + 14.7 psia = 42.6 psiaThe next step is to calculate the inside diameter of the relief vent pipe:
D = 1.5 inches - 0.133 inches (wall thickness of Schedule 40 steel pipe) = 1.367 inchesFinally, we can substitute all of the known values into the equation to calculate the maximum length of the relief vent line:
L = 9 * 42.6^2 * 1.367^5 / 100^2 = 272 feetTherefore, the maximum length of the relief vent line is 272 feet.
اندازه گیری شیر تسکین دهنده
شیرهای تسکین دهنده سیستم مایع و شیرهای کمکی به سبک فنری:
$$ A=\frac{G P M \times \sqrt{G}}{28.14 \times K_B \times K_V \times \sqrt{\Delta P}} $$شیرهای کمکی سیستم مایع و شیرهای کمکی با کارکرد پایلوت:
$$ A=\frac{G P M \times \sqrt{G}}{36.81 \times K_V \times \sqrt{\Delta P}} $$شیرهای تسکین سیستم بخار:
$$ A=\frac{W}{51.5 \times K \times P \times K_{S H} \times K_N \times K_B} $$شیرهای تخلیه سیستم گاز و بخار (Lb./Hr.):
$$ A=\frac{W \times \sqrt{T Z}}{C \times K \times P \times K_B \times \sqrt{M}} $$شیرهای تخلیه سیستم گاز و بخار (SCFM):
$$ A=\frac{S C F M \times \sqrt{T G Z}}{1.175 \times C \times K \times P \times K_B} $$Definitions:
- A: Minimum required effective relief valve discharge area (square inches)
- GPM: Required relieving capacity at flow conditions (gallons per minute)
- W: Required relieving capacity at flow conditions (pounds per hour)
- SCFM: Required relieving capacity at flow conditions (standard cubic feet per minute)
- G: Specific gravity of liquid, gas, or vapor at flow conditions (water = 1.0 for most HVAC applications; air = 1.0)
- C: Coefficient determined from the expression of the ratio of specific heats (C = 315 if value is unknown)
- K: Effective coefficient of discharge (K = 0.975)
- KB: Capacity correction factor due to back pressure (KB = 1.0 for atmospheric discharge systems)
- KV: Flow correction factor due to viscosity (KV = 0.9 to 1.0 for most HVAC applications with water)
- KN: Capacity correction factor for dry saturated steam at set pressures above 1500 psia and up to 3200 psia (KN = 1.0 for most HVAC applications)
- KSH: Capacity correction factor due to the degree of superheat (KSH = 1.0 for saturated steam)
- Z: Compressibility factor (Z = 1.0 if value is unknown)
- P: Relieving pressure (psia) (P = set pressure (psig) + overpressure (10% psig) + atmospheric pressure (14.7 psia))
- ∆P: Differential pressure (psig) (∆P = set pressure (psig) + overpressure (10% psig) − back pressure (psig))
- T: Absolute temperature (°R = °F + 460)
- M: Molecular weight of the gas or vapor
Relief Valve Sizing Notes:
- هنگامی که از چندین شیر کمکی استفاده می شود، یک شیر باید در حداکثر فشار کاری مجاز یا کمتر از آن تنظیم شود و شیرهای باقیمانده ممکن است تا 5 درصد بیش از حداکثر فشار کاری مجاز تنظیم شوند.
- هنگام اندازه گیری چندین شیر کمکی، کل مساحت مورد نیاز بر روی فشار بیش از حد 16 درصد یا 4 Psi محاسبه می شود که هر کدام بیشتر باشد.
- برای بخار فوق گرم، می توان از مقادیر ضریب تصحیح فهرست شده در زیر استفاده کرد:
Superheat Calculator
Selected Superheat: 0 °F
Correction Factor: 0.97
ارزش سوپرهیت | ضریب تصحیح |
تا دمای 400 درجه فارنهایت گرم کنید | 0.97 (محدوده 0.979-0.998) |
تا دمای 450 درجه فارنهایت گرم کنید | 0.95 (محدوده 0.957-0.977) |
تا دمای 500 درجه فارنهایت گرم کنید | 0.93 (محدوده 0.930-0.968) |
سوپرگرم تا 550 درجه فارنهایت | 0.90 (محدوده 0.905-0.974) |
تا دمای 600 درجه فارنهایت سوپرگرم کنید | 0.88 (محدوده 0.882-0.993) |
تا دمای 650 درجه فارنهایت سوپرگرم کنید | 0.86 (محدوده 0.861-0.988) |
تا دمای 700 درجه فارنهایت سوپرگرم کنید | 0.84 (محدوده 0.841-0.963) |
تا دمای 750 درجه فارنهایت سوپرگرم کنید | 0.82 (محدوده 0.823-0.903) |
سوپرگرم تا 800 درجه فارنهایت | 0.80 (محدوده 0.805-0.863) |
سوپرگرم تا 850 درجه فارنهایت | 0.78 (محدوده 0.786-0.836) |
تا دمای 900 درجه فارنهایت گرم کنید | 0.75 (محدوده 0.753-0.813) |
تا دمای 950 درجه فارنهایت گرم کنید | 0.72 (محدوده 0.726-0.792) |
سوپرگرم تا 1000 درجه فارنهایت | 0.70 (محدوده 0.704-0.774) |
Material Properties
Properties:
Molecular Weight:
Ratio of Specific Heats:
Coefficient C:
Specific Gravity:
You may use table instead of calculator
گاز یا بخار | وزن مولکولی | نسبت گرمای خاص | ضریب ج | گرانش مخصوص |
استیلن | 26.04 | 1.25 | 342 | 0.899 |
هوا | 28.97 | 1.40 | 356 | 1.000 |
آمونیاک (R-717) | 17.03 | 1.30 | 347 | 0.588 |
آرگون | 39.94 | 1.66 | 377 | 1.379 |
بنزن | 78.11 | 1.12 | 329 | 2.696 |
N-بوتان | 58.12 | 1.18 | 335 | 2.006 |
ایزو بوتان | 58.12 | 1.19 | 336 | 2.006 |
دی اکسید کربن | 44.01 | 1.29 | 346 | 1.519 |
دی سولفید کربن | 76.13 | 1.21 | 338 | 2.628 |
مونوکسید کربن | 28.01 | 1.40 | 356 | 0.967 |
کلر | 70.90 | 1.35 | 352 | 2.447 |
سیکلوهگزان | 84.16 | 1.08 | 325 | 2.905 |
اتان | 30.07 | 1.19 | 336 | 1.038 |
الکل اتیلیک | 46.07 | 1.13 | 330 | 1.590 |
اتیل کلرید | 64.52 | 1.19 | 336 | 2.227 |
اتیلن | 28.03 | 1.24 | 341 | 0.968 |
هلیوم | 4.02 | 1.66 | 377 | 0.139 |
N-هپتان | 100.20 | 1.05 | 321 | 3.459 |
هگزان | 86.17 | 1.06 | 322 | 2.974 |
اسید هیدروکلریک | 36.47 | 1.41 | 357 | 1.259 |
هیدروژن | 2.02 | 1.41 | 357 | 0.070 |
هیدروژن کلرید | 36.47 | 1.41 | 357 | 1.259 |
سولفید هیدروژن | 34.08 | 1.32 | 349 | 1.176 |
متان | 16.04 | 1.31 | 348 | 0.554 |
متیل الکل | 32.04 | 1.20 | 337 | 1.106 |
متیل بوتان | 72.15 | 1.08 | 325 | 2.491 |
متیل کلراید | 50.49 | 1.20 | 337 | 1.743 |
گاز طبیعی | 19.00 | 1.27 | 344 | 0.656 |
اکسید نیتریک | 30.00 | 1.40 | 356 | 1.036 |
نیتروژن | 28.02 | 1.40 | 356 | 0.967 |
اکسید نیتروژن | 44.02 | 1.31 | 348 | 1.520 |
اکتان N | 114.22 | 1.05 | 321 | 3.943 |
اکسیژن | 32.00 | 1.40 | 356 | 1.105 |
N-پنتان | 72.15 | 1.08 | 325 | 2.491 |
ایزو پنتان | 72.15 | 1.08 | 325 | 2.491 |
پروپان | 44.09 | 1.13 | 330 | 1.522 |
R-11 | 137.37 | 1.14 | 331 | 4.742 |
R-12 | 120.92 | 1.14 | 331 | 4.174 |
R-22 | 86.48 | 1.18 | 335 | 2.985 |
R-114 | 170.93 | 1.09 | 326 | 5.900 |
R-123 | 152.93 | 1.10 | 327 | 5.279 |
R-134a | 102.03 | 1.20 | 337 | 3.522 |
دی اکسید گوگرد | 64.04 | 1.27 | 344 | 2.211 |
تولوئن | 92.13 | 1.09 | 326 | 3.180 |
FREQUENTLY ASKED QUESTIONS
The required relief valve orifice area can be determined using the API 520/521 equations, which take into account the valve’s flow coefficient, the relieving pressure, and the required flow rate. The orifice area is typically calculated using the following equation: A = Q / (CKP), where A is the orifice area, Q is the required flow rate, C is the flow coefficient, K is the valve’s discharge coefficient, and P is the relieving pressure.
The relief valve vent line maximum length is critical because it affects the valve’s ability to relieve pressure safely and efficiently. A vent line that is too long can lead to excessive backpressure, which can prevent the valve from opening fully or cause it to reseat prematurely. The maximum length of the vent line can be calculated using the equation provided in the API 520/521 standards, which takes into account the valve’s set pressure, the vent line’s diameter, and the density of the fluid being relieved.
Selecting the correct relief valve for your application involves considering several factors, including the system’s design pressure, the relieving pressure, and the required flow rate. You should also consider the type of fluid being relieved, as well as any specific regulatory requirements or industry standards that apply. Other factors to consider include the valve’s material construction, its flow characteristic, and its certification or approval by relevant authorities.
There are several types of pressure relief valves available, including spring-loaded valves, pilot-operated valves, and rupture discs. Spring-loaded valves are the most common type and are suitable for most applications. Pilot-operated valves, on the other hand, are typically used for high-flow applications or where a high degree of accuracy is required. Rupture discs are used in applications where a rapid release of pressure is required, such as in fire suppression systems.
Proper installation and maintenance of pressure relief valves are critical to ensure their safe and efficient operation. Installation should be carried out in accordance with the manufacturer’s instructions and relevant industry standards. Regular maintenance should include inspections, testing, and cleaning of the valve to ensure it remains functional and free from blockages or corrosion.
Inadequate pressure relief valve sizing can have serious consequences, including equipment damage, injury to personnel, and environmental harm. Undersized valves may not be able to relieve pressure quickly enough, leading to a buildup of pressure that can cause catastrophic failures. Oversized valves, on the other hand, can lead to excessive flow rates and energy losses. Proper sizing of pressure relief valves is therefore critical to ensure safe and efficient operation of process equipment.
